...Philosophers asked the Jewish Sages in Rome, “If G-d is not favorably disposed toward an idol, why does He not simply get rid of it?”
The Sages answered, “If people worshipped something that the world does not need, G-d would indeed get rid if it. But behold, they worship the sun, the moon, the stars - should G-d destroy the world because of fools?”
The philosophers then said, “If so, let Him destroy every idolatrous object that is not necessary for the world.”
The Sages answered, “Then an argument could be made that we are encouraging the worshippers of the sun, moon, etc., for these worshippers will say that theirs' are the true deities, for they were not destroyed while other forms of idolatry have.”
http://community.livejournal.com/talmud_daily/139049.html (Avodah Zarah 54)