Why was consumption of animal products forbidden before the Mabul?

Rashi on Bereshit 9, 3; brings the Talmudic teaching that before the flood man was not permitted to kill for meat and only after the flood it was permitted. The commentators (R. Bachye, Meshech Chochmah) explain it to be a reward to Noah for caring for them during the flood, and for the fact that if not for Noah – no animals would have survived. (As for fish – they were permitted so there'll be no difference between the species).