The case of the person identified in the OP is not clear cut. He is working as an expert consultant rather than a temporary worker. By the letter of many state laws, he would have to be licensed. However, the industries that he serves are mostly manufacturing. Manufacturing is international in nature and not state specific, like a building or piece of infrastructure. As such, it is not really feasible for someone in such an industry to pursue state licensure. California, for example, has recognized this situation and grants license exemption to those who consult in regard to manufactured products. Most likely other states have lagged behind on this issue and their laws are "behind the times". But in any event, licensure is legally only required for a small percentage of engineers. Others (like myself) get it as a credential.
BRGENG, Your view of medicine is from a generation ago. Insurance companies (and business type doctors/administrators) have made the lives of regular doctors a nightmare. After the war stories I have heard from doctor friends, I have concluded that engineers (PE or not) are far more professional than folks in the medical field.