As I recall from my younger days, you had to be in pretty good condition to maintain 20mph using pedals.
Sure, any bike can go more than 15mph down a steep hill, and a powerful rider can go 30mph for a good stretch... but a bike that goes >15mph regardless of rider, is trouble in the making; or a re-writing of all the bike laws in the country to restrict some sales to adults. As you say, they might as well be motorcycles at that point.
The conventional bike itself has an inherent restricting feature: a 13yr old can't make it go very fast for very long. Slap a motor in it and you got a motorcycle... unless it's a puny little motor, then you got a moped or an e-bike.
Regulations protect the builders/retailers too:
If I'm a dimwitted parent at Wally-world and the clerk says: "that's a motorcycle"... I see that my kid shouldn't ride it until age 16 (or whatever). If I give it to my spoiled 12yr old anyway and he/she gets hurt... Wally-world is in the clear.
If my smarter brother sees an e-bike at Wally-world and the clerk says: "that's an e-bike"... he knows its safe enough for his kid to operate. If his genius 10yr old kid slaps an e-tek and 50lbs of lipos in the rig and jumps over eight lanes of interstate, Wally-world is still in the clear.
As a supplier or a manufacturer, are you going to take a chance that Wally-world won't stock your units because they get too close to the line? Not if you like making money... you are probably going to make sure every potential retailer knows your units can be handed to an unlicenced kid and you/they won't have to "stand-up" in a civil action, even if the kid adds a WileECoyote booster.
Just like "replica" or demilitarized weapons, anybody can have a retail item converted to a more dangerous (and perhaps illegal) item. The responsibility is then upon the owner to comply with restrictions that apply to the class of item the conversion falls under.