User blog comment:Deshiba/Taunt rework/@comment-24674556-20150604215934

I think the reason to why Shen's and Galio's taunts make the targets deal less damage to them has more to do with them getting tougher, rather than the enemies getting weaker.

But why won't make these two ACTUALLY getting tougher? Well, because the intention is just to affect the enemies taunted, giving a chance for reaction from the other members. It's "easier" to apply a damage debuff to the targets than giving Shen and Galio %damage-reduction-that-only-applies-to-taunted-enemies.