It must be the different versions thing. This is what my version says on the subject.

"But Apollo would not suffer it to be disfigured, for he pittied the man, dead though he now was; therefore he shielded him with his golden aegis continually, that he might take no hurt while Achilles was dragging him."

Also, considering that Achilles dragged him face downward, I'd think it'd be a bit harder for his hair to become sullied.