When you shoot a mob that has armor with thorns enchantment, you still may be hurt.
Also, if you have armor with thorns and skeleton shoots to you, skeleton will sometimes receive thorns damage.

I think it doesn't work as intended, because thorns enchantment should only affect melee combat.

To reproduce:
1. Equip armor with thorns enchantment
2. Be hurt by skeleton's arrows
3. Skeleton will sometimes receive thorns damage (for thorns VII chance is 100%)