I'd bet money that most people that vegan or vegetarian are also far more active about human suffering. It makes sense that someone that believes we should (and can) act to reduce/end suffering would apply that concept universally, rather than selectively.

But just so you realize, your argument sounds EXACTLY like all the right wingers that suddenly care about all the poor americans once immigrants enter the discussion. Most people that make these arguments care about neither, all they care about is not having to actually do anything and still feel moral.