There's a fundamental relationship at work here which probably guides the lawmakers.
The performance of an unpowered bike is directly linked to the health and fitness of the rider. Those capable of the highest performance are likely to have the fastest reactions, the greater awareness and the best eyesight, all usually through being more youthful.
Conversely a power assisted bike providing as much as four times the rider's effort in addition can greatly exceed the safe capabilities of someone far less well endowed through age or unfitness.
That of course is true of all powered vehicles, hence them requiring operator testing, licencing, insurance and traceability. You can have more power assisted speed legally, even more power, but they come with those impositions for good reasons.
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