I hadn't really given it a second thought and assumed that they were legal to ride just as a pedal bike. Appears I was wrong.
MASSACHUSETTS’ E-BIKE LAW FOR THE ROAD » In Massachusetts, an e-bike is defined as a “motorized bicycle” as long as its maximum speed is 25mph. As motorized bicycles, e-bikes are not subject to the same rules of the road as regular bicycles. » E-bike riders must carry an operator’s license and are subject to registration requirements. E-bikes are not subject to insurance requirements.
» Helmets are required, and there is a 16 year age minimum for e-bike use. E-bikes are not allowed on sidewalks or bike paths.