It doesn't look to me like he actually made physical contact with any part of Henne above the shoulder pads, and that Henne's head snapped back from the force of impact of the hit rather than from direct contact. Nevertheless, coming in that high and leading with the helmet is going to draw a penalty every time in this NFL, right or wrong. If the goal is to protect the QB, coming in that high is getting into dangerous territory where you can cause a debilitating injury. If that had been Brady getting hit, I think most of us would have wanted a penalty.
I think it was a clean hit, and a great hit, but a dumb hit to go in that high. But the NFL certainly isn't making it easy for defenders. It's sort of like a "strike zone" in baseball for a legal hit these days - above the knees and below the numbers - but with a moving target.