If I might adapt my grandfather's passed down driving instructions:

when it's passing more pedal bikes than pass it.

It's really that simple - if the motor is letting someone fit in, it's a good thing. But once it's leading them to stand out, it's a negative for society to continue to pretend that it is any sort of "bicycle".

Such motorcycles belong on the ordinary motor vehicular infrastructure and the ordinary motor vehicle regulations, if they displace internal combustion versions there, all the better.