I mean, the vegans in question are also human (one presumes), so it makes sense they consider they have agency over themselves, not - for instance - over a lion which might ought to make up its own mind. And vegan humans might try and persuade other humans to embrace vegan ethics without yet making the wider step of preaching to lions (who might complain of colonialism or cultural dominance).

One presumes vegan lions, if such were to exist, would argue against the exploitation of humans for food (as in zoos), without requiring humans to conform to the same ethical standard.