If this is a feature, I don't know why. If you get hit by a skeleton/other mob by your sitting cats, they will stand up till the monster is killed. I don't know why the cats would stand up. It's not like they chase the monster away.

Sidenote: I can push my cat into a wall and it will suffocate and die. I have just lost 8 cats in the last 10 minutes because of this. It happens also when they grow up.