Why was fortune telling through Astrology, Astromancy, Face Reading, Palmistry, Cartomancy (and a number of other methods) forbidden by the Rabbis, when there is numerous instances in the Bible of the Prophets (during the period of Prophets/Nevuah) doing exactly that (prophesying)?
The Torah is quite explicit about fortune telling:
When you come into the land which the LORD your God is giving you, you shall not learn to follow the abominations of those nations. There shall not be found among you anyone who makes his son or his daughter pass through the fire, or one who practices witchcraft, or a soothsayer, or one who interprets omens, or a sorcerer, or one who conjures spells, or a medium, or a spiritist, or one who calls up the dead. For all who do these things are an abomination to the LORD, and because of these abominations the LORD your God drives them out from before you. — Deuteronomy 18:9 (NKJV)
And yes, many of the Prophets did prophesy (who would have guessed?). But they did so by consulting the Lord directly, not by using mystical mechanisms:
And when they say to you, “Seek those who are mediums and wizards, who whisper and mutter,” should not a people seek their God? … — Isaiah 8:19 (NKJV)
Did the Prophets under their own initiative use magic to find the truth, or was it the Lord himself that chose to reveal the truth to them?
See Devarim 13:1-5: "If a prophet, or one who foretells by dreams, appears among you and announces to you a miraculous sign or wonder, and if the sign or wonder of which he has spoken takes place, and he says, 'Let us follow other gods' (gods you have not known) 'and let us worship them,' you must not listen to the words of that prophet or dreamer. The Lord your God is testing you to find out whether you love him with all your heart and with all your soul. It is the Lord your God you must follow, and him you must revere. Keep his commands and obey him; serve him and hold fast to him. That prophet or dreamer must be put to death, because he preached rebellion against the Lord your God, who brought you out of Egypt and redeemed you from the land of slavery; he has tried to turn you from the way the Lord your God commanded you to follow. You must purge the evil from among you"