Eh, don't forget that humans often hate other humans. Exposing an anti-vegetarian to vegetarian advertisements might induce them to increase their meat intake, and an annoying advocate may move someone from neutral to anti-vegetarian.

If you take a non-vegetarian and make them more non-vegetarian, I don't think much is lost, because you never would have captured them anyway. I suppose they might eat more meat or try and persuade other people to become anti-vegetarian, but my intuition is that this effect would be really small.

But you're right that it would need to be considered.