The Thorns enchanment, at least Thorns II seems to be having the opposite effect.

I enchanted a iron chest through an enchanting table with 30 levels and got Thorns II Unbreaking III and Blast Protection II. Upon hitting mobs I would occasionally take the thorn damage myself, Instead of them hitting me and taking the damage.
Steps to Reproduce:
1. Enchant iron chest wiith Thorns II
2. Hit mobs
3. Take damage.
3. ...