To answer you in the most simple way possible, Pedram:
We need to use the gerund (teas-ING) because it is an action that those kittens are doing at that very moment. Whilst they are doing this action (continuously) they are not happy.
I hope that this helps.
Another example would be: Kittens that tease balls of string do not have fun.
This makes it a statement about all kittens at any time.