They might not need to eat a lot, but presumably they need to eat at least enough to keep from starving to death. (Although it would be cool if they could live off the True Power). No info has been given so far in the books as to how much a trolloc actually eats, though. We also have no info on how much food really can be found in the blight (I'm pretty sure RJ said something once about blight-resistant crops), and we don't know how many thousands of trollocs are actually being gathered there. Need more info! Still though, given that the blight is a wasteland and probably not very productive even using resistant crops, and given that BS needs enough trollocs to make the LB an Epic Battle(tm), it seems reasonable that the number of trollocs would be over the carrying capacity of the land and that they would have to be importing food somehow, even if they're being kept on a starvation diet.

Plus, they're presumably making loads of Myrdraals too, and they would still need lots of warm bodies to kill to make their fade-swords. Unless they're using darkfriends for that.