"You could have a 1500 watt limit for both and make life a lot easier and simpler for heavy riders, cargo bikes, etc. with little or no impact on safety."...there most definitely would be an impact on safety, the motor and battery allowing 1500w output would be something on the order of six times heavier (about a hundred pounds) than a 250w setup (given the same "marketing" range per charge) Impact energy goes way up, even at 20mph, if you add 30% to the bike/rider weight. You need better brakes, better tires...
And once you allow 1500w, you'd need firmware in the controller to to keep the bike below the legal speed limit, and soft start programing to keep the driveline happy and the tires from slipping under low traction conditions. How long until those limits were bypassed by "performance" riders? Some sort of hidden switch or smartphone app, so an observant LEO couldn't duplicate the extralegal behavior he just witnessed.
It wasn't uncommon, up until 40 years ago, to have ICE vehicles taxed based on their displacement or horsepower, so it's not at all out of the realm of legal precedent to put legal limits on power output for a given class of vehicle.
All of this argument is about wanting to be able to call small electric motorcycles "bicycles", and sidestep sane restrictions on motor vehicles.