Why did God make all plants, but only some animals, kosher? Sources, please.
The famous saying "you are what you eat" may explain this. Plants do not tend to have any unruly behavior. However there are many animals that kill other animals and behave - in lack for a better term - animalistic. Hashem does not want us to eat from these animals, in order that it should not make us behave in the way they behave.
There are actually many laws that apply to plants, too, to make them "kosher", but they are all connected with/ apply in the Land of Israel. What makes a plant "kosher" is not the specific species of plant (which is what determines the kashrut of something from the animal kingdom), but how and where it was grown, and the laws that apply to it once you want to use it. All of these laws fully apply in the Land of Israel when there is a Temple standing. Some apply (sometimes in a more limited way) at all times in the Land of Israel. And a few (like orlah, challa, chadash according to some opinions) apply outside Israel too. (Some are rabbinic rather than Torah law today.)
- Fruit grown on trees that are under three years old are called orlah and are forbidden to be eaten. The fruit of the fourth year must be eaten under conditions (that we don't have now) in Jerusalem or redeemed.
- All produce grown in the Land of Israel must have tithes given to the priests and Levites and the poor (truma and trumat maaser, maaser sheni and maaser oni), and these laws still apply today.
- There are laws regarding the shemitta (sabbatical) year, and produce grown in a prohibited way during the sabbatical year may also be prohibited. Fruits that grow by themselves have holiness (kedushas shviis) and are subject to particular laws (not to waste them, to use them in the usual way, not to buy and sell them, etc.)
- A portion of our dough (challa) is given to the priest (kohen). Since the kohen cannot eat it today (because he cannot become ritually pure), it is separated and burned. But dough that didn't have challa separated is non-kosher.
- The "new" grain (chadash) cannot be eaten until the omer offering is harvested on the 2nd night of Pesach (Passover). Today, we must still wait for the 2nd day of Pesach until eating new grain.
You won't find that YHWH declared ALL plants "kosher" in Scripture at any point in time, even before the curse. You're forgetting the very first plant He ever prohibited: the tree of the knowledge of good and evil.
But not to jump too far ahead, according to Genesis (or Bereshiyt) 1:29-30, YHWH gave criteria that defined what plants mankind could eat as food out of all the plants that He created. So, YHWH did not create ALL plants to be eaten by us. This applies across the board—not just to Israel, but to all human beings since we're talking about the first human pair, Adam and Eve, in Genesis 1-3.
Genesis 1:29-30 (NIV)
29 Then God said, “I give you every seed-bearing plant on the face of the whole earth and every tree that has fruit with seed in it. They will be yours for food. 30 And to all the beasts of the earth and all the birds in the sky and all the creatures that move along the ground—everything that has the breath of life in it— I give every green plant for food.” And it was so.
According to those verses, the plants that YHWH designed for us (mankind) to eat were plants that bore seeds—or trees that bore fruit with seed in it; in contrast, every green plant was given to the animals (not to us).
To provide a practical example of this distinction: animals, like rabbits and deer, can safely eat grass and benefit from it; we can't.
Most herbivores, such as cows buffaloes, goats, sheep and deer, eat grass. Animals that live in the forest eat grass, twigs, plants and leaves. Horses eat hay, which is dry grass. Even insects such as grasshoppers eat grass. Elephants love to eat the bark of trees, but even elephants eat grass. [...]
In principle, people can eat grass; it is non-toxic and edible. As a practical food source, however, your lawn leaves a lot to be desired.
There are two main problems with a grass diet. The first is that human stomachs have difficulty digesting raw leaves and grasses. Animals such as cows, on the other hand, have a specialized stomach with four chambers to aid in the digestion of grass (a process called rumination).
Aside from the digestion issues, a second problem with grass as a food source is the mastication. Your dentist would not be pleased; grass contains a lot of silica, an abrasive which quickly wears down teeth. Grazing animals have teeth that are adapted to continually grow, replacing the worn tooth surfaces quickly.
So, no, not all plants should be eaten by man (ergo, not all plants are "kosher"; I'm assuming we're using "kosher" in the way that YHWH, through Moses, used the term "tahor" [טָהוֹר] H#2889, in Leviticus 11:47, to distinguish the living creatures we may eat in contrast to the "tame"[טָמֵא] H#2931, the living creatures we may not eat).
- Leviticus 11:47 (NIV)
47 You must distinguish between the unclean and the clean, between living creatures that may be eaten and those that may not be eaten.’”
"Kosher"—a term that does not appear in the written Torah—literally means "clean / pure". But the Hebrew term that appears in Leviticus 11, in the written Torah, is "tahor". "Tahor" (clean) applies to "what may be eaten". Even though the terms "tahor" (clean) and "tame" (unclean) are not used of plants, the concept that it represents ("may be eaten" from "what may not be eaten") is: some plants may be eaten, some plants may not eaten. Ergo, not all plants may be eaten by man, just like not all animals may be eaten by man.
Technically, all plants and animals CAN be eaten (not may, not should, but can be eaten) by man—you can chew and swallow them—but that doesn't mean you should; YHWH sees a negative repercussion that we can't see; so, if it's not conducive to our good, in the immediate or in the long run, then YHWH Commanded against eating it. For the same reason, He set the distinctions for other things besides plants and the meat of animals:
Out of all the criteria YHWH gave us, for our own good, we also must not eat blood (Leviticus 17:13-15), and not surprisingly blood is a carrier for pathogens; the fat is also prohibited (Leviticus 3:17; 7:24), not surprisingly, toxins get stored in the fat; the liver and kidneys and the fat surrounding these organs are also prohibited (Leviticus 3:4), and not surprisingly, these organs filter out the toxins in the body [thus, there is toxin in that organ]. Of what is not mentioned explicitly, but is obvious (in our day anyway, can't speak for future generations): we also must not eat poop, nor wood, nor stones. Just because YHWH created it and it's not an animal, that doesn't mean we can eat it. Those aren't "food" (a thing to be eaten) either.
After the fall, things like thorns and thistles came up from the ground (I assume poison ivy arose at this time too, considering that it has berries [fruit] with seed in it, thus fits Genesis 1 criteria, but is poisonous to us; ergo is not a part of YHWH's original "very good" creation in the beginning, but a result of the cursed earth i.e. Genesis 3:17-19). So we can't go by Genesis 1 criteria alone (though Genesis 1 criteria already had its own limits too, but once man sinned, more limits were placed because we messed creation up to our own discomfort and detriment).
Genesis 3:17-19 (NIV)
17 To Adam he said, “Because you listened to your wife and ate fruit from the tree about which I commanded you, ‘You must not eat from it,’
“Cursed is the ground because of you; through painful toil you will eat food from it all the days of your life. 18 It will produce thorns and thistles for you, and you will eat the plants of the field. 19 By the sweat of your brow you will eat your food until you return to the ground, since from it you were taken; for dust you are and to dust you will return.”
In short: the question you asked assumes something to be true that is actually not true at all: YHWH did not say ALL plants were "kosher" (nor that all plants were "tahor"), clean, "may be eaten". YHWH Commanded that only the seed-bearing plants and the trees that bear fruit with seed in it may be eaten—with the exception of the tree of the knowledge of good and evil, which was totally forbidden. And after Adam and Eve's sin, there were even more plants that we could not eat of because of the cursed ground. Just like some animals are off-limits, some plants are off-limits too—for our own good.