For these "fatbikes" they are cheap non-conpliant e-mopeds which are used by schoolkids who really do not have a clue what the risks are riding how they do. It's not just Amsterdam, in the east there are a few citys who banned all e-bikes from the city center due to these "fatbikes". So there are a few legal fatbikes riding around (which are way more expensive than these china-schrott versions) and these are ok as they are compliant, the problem bikes are these cheap "fatbikes", these cost less than a proper e-bike battery and they have a to powerfull motor and a throttle. Also these are a danger to use in public, two years ago a young kid with his friend on the back ran into a mother and toddler (who where riding a Giant e-bike with a bobike seat), the Giant had a broken front wheel, the "fatbike" was in 3 pieces (frame snapped) and both kids needed to go to the hospital, mother and toddler where just brushed and could walk away.
For these "fatbikes", they are more e-mopeds and if they where on a licenceplate the rider would have to be older than 16 and have insurance, licencse, helmet etc. Since they are bought online and with a simple button combination are unlocked there are kids from the age of 8 riding these things.