In both empowered and normal modes, the RF crook uses just item durability without using RF. The crook seems to functionally work on leaves (drops silkworms) but does nothing to animals.

Out of curiosity, what should it do on animals?

The (shepherd's) crook is used to pull (herd) animals towards the player with right-click. The regular Ex Nihilo wooden and bone crook take normal durability when shepherding animals and double durability on leaves (a secondary function).

To the durability problem: All R.A.'s RF tools take durability damage (except the wrench which doesn't have durability), so it seems the implementation is wrong (maybe?) in the main mod as well.

Hope that helps.