The bike I'm buying and most of the nice ones in the USA are powered by the Yamaha and Boshe systems which met the EU standard:
Cycles with pedal assistance which are equipped with an auxiliary electric motor having a maximum continuous rated power of 0.25 kW, of which the output is progressively reduced and finally cut off as the vehicle reaches a speed of 25 km/h (16 mph) or if the cyclist stops pedaling."
Except that cutoff is at 20mph.
This despite that US law:
"low speed electric bicycle" as a two or three wheeled vehicle with fully operable pedals, a top speed when powered solely by the motor under 20 mph (32 km/h) and an electric motor that produces less than 750 W (1.01 hp)" By comparison my KTM makes 50HP.
However USFS and BLM do not yet acknowledge the existence of these vehicles at all. Since there is such high sensitivity, I would not be against using the euro version of the definition with 20mph limit, because those bikes are easy to buy. They exist today in the USA. I don't think the 750w would really make a real world difference in user/user interaction, but you and I'm sure others do.
So, I would compromise