It's called being a cynical asshole. Or sometimes people feel like they'll be popular if they join the hate crowd.
A lot of people hate Blizzard. Most of the time it's for no good reason (i.e: THIS GAME IS SHITTY!!! <.>). So, whenever Blizzard announces something or changes something, these people are thinking "Ok, so how are they gonna fuck this one up?" and by looking at things that way all the time they find the most ridiculous things to pick at like pet battles. It doesn't affect them yet they complain about it like it's the end of the world.
Other people aren't as creative so they need to jump on the hate bandwagon. A blizz-hater see's the expansion announcement and can't find anything stupid enough to pick at. He see's someone else complaining about Kung fu panda even though the timing doesn't line up (warcraft 3 came first), they all start complaining about it. Some people get really freakin crazy about it, too.
I won't say any names, but yesterday while I was re-installing everything after reformatting my hard drive all day, I noticed this ONE user on MMOchamp in EVERY MoP thread linking an image of Kung Fu Panda and insulting blizzard. In response to people telling him facts, he called them moronic blizz fanboys. There's nothing you can do to stop these cynical, ignorant, lifeless haters. No matter what the topic is, people always find something to complain about.
:\