I think a lot depends upon interpretation.
Leading with the helmet to me indicates lowering of the head to strike with the crown. Back in the day was called spearing. Harrison's hits have not been this.
As the one on McCoy, his head is clearly up. McCoy lost the protections given a QB while in the pocket as a 'defense-less' player when he scrambled, upon which he is treated like any ball carrier. He was not exposed like a WR making a catch. There would be no issues had Harrison hit a RB coming out of the backfield.
Harrison has his head up on nearly all of his hits. Things is, he is like a 5'11" linebacker, so he always plays pretty low & comes up into ball carriers, etc. Tackling nowadays is being taught to wrap up & initiate contact with your chest. However, when Harrison does this it results in his head sliding up from the opponents chest & his facemask or helmet nails the ball carrier in the chin/facemask/helmet.
Technically, I think the facemask is part of the helmet when determining 'helmet to helmet' contact. Just think in this last case w/McCoy that it applies.
Yes, I am a Steelers fan, but some times Harrison deserved the fines, etc. However, I agree in this case that he should not be expecting a fine. Will likely receive one as he has not done himself any favors with Goodell & the NFL.