I’m just guessing, but I think it’s because with a throttle it would be considered a “snorfiets” (light moped with a maximum speed of 25 km/h), which would mean:
- minimum age of 16
- license plate
- insurance
- drivers license
- different traffic rules (mostly the same as bicycles, as opposed to 45 km/h mopeds)
I think they wanted to make electric bicycles with assisted pedaling an attractive alternative to noisy and polluting petrol fueled mopeds.
Also, we’re talking about a country with more bicycles than citizens, so treating them as “snorfiets” would mean that a lot of cyclist would be forced to share the road with faster motorized vehicles on some stretches of road (though not many), which would create other safety issues.
I’m not sure if I ever heard that one, but that’s indeed nonsensical. ISO 216 is a very nice system, though, and used in most of the world, not just Europe.