In the US, it is not required for the operator of a bicycle to be licensed anywhere. In a few places, the bike has to be licensed, but no testing of the rider's ability or knowledge.
The issue with speed of an ebike is simple. At some point, you have a vehicle capable of speeds and cargo capacity that essentially all you're doing is slapping a pair of pedals onto a motorcycle in order to get around drivers license and vehicle inspection requirements. We're designating a low-powered class of vehicles as not being motor vehicles, but that has to cut out somewhere. 30 mph is an arbitrary point, but so is any other number.