I think it is somewhere between not wanting to/can't look him in the eye and it would freak Howard out. Does he harbor some sort of guilt - thinking he should have done something? Maybe. But he will push those emotions way down and I do not think he would ever admit to it. We know its not a "Sirius will not let him in the building" issue. Interesting when the book comes out - will he be pressured when Artie is on every other show? Will he make him phone in? Poor Howard is going to have to deal with it soon enough.