Major problem in the U.S. too.

Easily modified to go as fast as 50 MPH on a chassis not designed for it. Drivers aren’t licenced and often are young kids. No registration. No insurance. No training. Very hazardous to pedestrians.

Oh, you mean they're powered, right?

Yes- in the Netherlands the term 'fatbike' is pretty much synonymous with the battery powered bikes only (I presume elsewhere this may be different). They are mini motorcycles really- but exempt from all the rules and regulations that would apply to regular motorcycles.

Elsewhere “fatbike” just refers to the tyre size.

Pedal fatbikes for riding on snow and sand have existed for at least 20 years I’d say.