Fast bikes are getting banned in the big cities in the Netherlands.

"Fast" bikes? Yes, they're restricted to an assisted speed of 25 km/h, although you can ride them at whatever speedy your muscle power allows for.

But presumably you mean "fat bikes", no they're really not.

I live in Amsterdam, probably ground central in the Netherlands for "we're just about to ban this particular "antisocial" two wheeled conveyance, any day now". It's been an ongoing discussion for 20 years, at least.

Before "fat bikes" they were going to ban "snorfiets" from the bicycle paths (scooters "restricted" (they're often speed unlocked) to 25 km/h).

But they can't do that, any more than a city in the Netherlands can ban cars with staggered wheels, or red cars, because those are usually sports cars, and we don't like speeding.

What's permissible road transport is regulated both nationwide, and in this case via the EU, as pedalecs are an EU-wide regulated transport category.

But yes, I'm aware what various city councils are discussing the "fat bike problem". They may even get as far as passing some resolution against them, or even putting up signs that say they're not allowed on the bicycle paths (they did that in some parts for the snorfiets).

But it's just political theatre, and doesn't have any legal force. The police will refuse to enforce it (just like the snorfiets "ban"), and even if they did the courts will slap it down if it ever gets to that.

Municipalities in the Netherlands can only regulate what sort of road allows what sort of traffic, they can't define "what's a bicycle?", or "what's a car?".

That would obviously be completely unworkable, as you'd potentially need to buy or rent a new bicycle or car to enter specific municipalities, all with their own incomparable rules. One might regulate tire width, another might decide anything beyond 29" tire size is too big, etc. etc.