Lots of things have a boundary, even if the boundary is somewhat arbitrary. In most places there is a voting age, there is an age of consent, and there is a drinking age. These ages differ, because there is no clear-cut boundary.
Do you think there should be no power limit, or that it should be a different power limit?
I don't see why the regulations for electric bikes should be much different from gasoline bikes (aka, mopeds). If your e-bike is powerful enough, then it's comparable to a more powerful moped.
Do you think we prevent people from drive on the sidewalk because they will be reckless? I think the greater the momentum disparity, the more likely it is that an error, like loss of control, results in injury or death. This is a wider brush than "reckless".
They don't bother with ebikes anyway, so they're reckless with a moped, which is way more dangerous.
The 250W limit in Europe is stupid (plus it shouldn't be a European law, it's not the correct scope).
Finally, you can be reckless with rollerblades or a bike, you don't need a motor. I guess it's more a problem of "250W is more than enough for people" just like 640kb was, for politicians.