Why is a non-kosher scroll (e.g. a photocopy of a kosher one) invalid, especially if the text is correct and looks right? Doesn't such a scroll still fulfil the purposes of serving as a reminder? And doesn't it fulfil just as well the biblical requirement to have those words on one's doorpost?
Doesn't such a scroll still fulfil the purposes of serving as a reminder?
And doesn't it fulfill just as well the biblical requirement to have those words on one's doorpost?
Rambam Tefillin, Mezuzah and Sefer Torah explains that the parchment and ink used for the tefillin, mezuzos, and sefer torah were specified by halacha leMoshe Misinai and if any of the requirements are skipped, then the result is invalid. In fact, if the scribe writes the names of Hashem without the proper intent, then it is invalid.
This means that each letter must be written individually. This invalidates methods such as a photostat or printing.
Just as meat slaughtered improperly, even if it is means having cut 49.9% of the esophagus and trachea instead of 50.0001%, is forbidden to eat, so too any torah, tefillin, or mezuzah that is written improperly is not kosher (and does not fulfill the mitzvah). The rules were set up by Hashem and given to Moses at Har Sinai. These laws were passed down without a break from Moshe through the chain of tradition until today. This is known as Halacha LeMoshe Misinai. Just because someone thinks that a reason still applies (even when the halacha is not followed), does not make it valid.
Your question What justifies the rules is really not a good way to express the question. These are the rules that hashem gave Moshe and that is all the justification that we need.
For example The Mezuzah Scroll and Case
The Mezuzah scroll is made from handmade parchment from a kosher animal and inscribed in black ink with a special quill pen. It is written by a specially trained, religious devout scribe, known in Hebrew as a sofer. The sofer concentrates intensely and writes with special Hebrew characters in a beautiful calligraphic hand.
A Mezuzah has 713 letters. Every letter has numerous laws pertaining to its form. In order for a Mezuzah to be written in accordance with all of the laws, it must meet thousands of requirements.
If even one of the Mezuzah’s 713 letters is missing, or shaped incorrectly, the Mezuzah is rendered invalid and the commandment is unfulfilled.
See a couple of Gemarot:
דתני רב יוסף לא הוכשרו למלאכת שמים אלא עור בהמה טהורה בלבד
for "heaven's work, skin of clean animal only is allowed
אמר ליה מנין שאין כותבין תפילין אלא על גבי עור בהמה טהורה אמר ליה דכתיב (שמות יג) למען תהיה תורת ה' בפיך מן המותר בפיך
We learn this from the verse which says "with the intention that Torah of God lies in your mouth"
תא שמע הלכה למשה מסיהי תפילין על הקלף ומזוזה על הדוכוסטוס קלף במקום בשר, דוכוסטוס במקום שיער
Halacha from Moses who received it at Sinai, ...mezuzah has to be written on some specific kind of scroll.
דתנא דבי מנשה כתבה על הנייר ועל המטלית פסולה, על הגויל ועל הקלף ועל הדוכוסטוס כשרה
If a mezuza is written on paper or tissue, it is not regular.
Why is a non-kosher scroll (e.g. a photocopy of a kosher one) invalid
The ultimate argument is that such was the transmission. We learn that specific part of skin is used because of their strength, and that for this heaven's work, only clean species are valuable because Torah has to do with mouth which is concerned by kashrut
A mezuzah is not only meant to be a reminder about Hashem, it is also a shmirah on the house. There seem to be differing opinions about which reason is the ikar (most important). See the Rambam in hilchos tefillin and mezuzah perek 5 halacha 4 where he excoriates people who put up mezuzos for the purpose of shmirah (see the Kesef Mishna there who asks from the gemara in Menachos 33b which seems to say outright that there is in fact a shmirah from the mezuzah, and he answers that according to the Rambam, the shmirah is only a secondary thing, not the main thing). The Rambam later, in perek 6 halacha 13 describes the purpose as being more of a reminder, ע"ש. However, Tosfos in Menachos 44a ד"ה טלית (about a third of the way down) describes the main purpose of the mezuzah as being the protection, and the same is mashma in Rashi Pesachim 4a ד"ה חובת הדר, as well as the Nimukei Yosef in hilchos Mezuza (in the backk of Menachos) 6b ד"ה משכיר.