I agree with pretty much what everyone said. Animal Rights and or Welfare issues are really tricky, because not only do you have to account for basic biology of humans, its delving a lot into the moral "unknowns" of animal psychology and, I hate to say emotions because that's the kind of language that makes people not take the cause seriously, but that's basically it.
Her whole response just kind of, in my mind, just pointed out that she isn't the best with "PR" of her fans. That's not to say that everyone should be a master mediator or anything, but it feels more like she is doing her best by trying to smooth out the anger in her fan base while still being "forceful" with her own beliefs. It might work for some fans, but for people who are thoroughly invested in the Animal Rights movement, or perhaps even people who payed any sort of "dues" in humanitarian causes, it's a huge slap in the face. (Though, very few people have at that age, and there's nothing wrong with that.) I guess KAA didn't want her series turned into propaganda, which is fair enough, its HER series.
I might be a little more sensitive just because I have put some (not as much as I should, but some) foot-work and volunteer hours into homeless animal rescue, and one of the really common arguments we come up against is "you're wasting time and money because you should be helping people" or "why don't you care about people this much, are you a psycho or something?" But overall, I agree with Phoenix, whether its environmental factors or just mental differences, different causes ARE going to mean more to you, and your time is best spent doing something you're passionate about instead of something that doesn't make you quite as happy.
But on the other hand though, I don't think the ghost-writer presented her case very well either. It was basically a book written by and for vegetarian-activists. No one else is going to care about the message because Ax musing "Gee, do chimps have citizenship" or "I wonder why humans hunt harmless animals" or Cassie going on a crusade (again...for the billionth time) doesn't have any sort of impact beyond being a little bit amusing or just plain annoying.