Your right, they are both classified as grief. The difference is, we can easily see logs of what changes were made to blocks and then have the system reverse the changes. With animals there are no logs and as such we can't reverse the changes. We also don't have the ability to create animals out of thin air.
Even if we could just create animals, we wouldn't be "allowed" to because if we were then anyone can simply say "Someone killed my _____" and get free animals. We have no way to prove if they had the animal or not. We can't though, so that's not a problem.
It's nothing against you personally Creepercicker. The one responsible will pay the price for what they've done, by being banned from the server. That's, unfortunately, the most we can do in the case of griefed animals.
My suggestion is to build a complete enclosure around any new animals you get, with PS protection, so griefers have absolutely no chance to kill them.
