@New-Blue-Dormouse-3474179314#276566 Banks have to adhere to certain regulation. They can make it so that a bank is also partly responsible for the behavior of its users. The bank therefore needs some way to ensure users are not using the services for illegal behavior. Depending on what kind of transaction has been done, this can be easier or harder.
A card transaction at your local Albert Heijn for example with a reasonable amount, there it's quite safe to say you're just grocery shopping. But what about 50.000 Euro incoming from a crypto exchange's bank account? It could be 100% legitimate, or maybe not. The only way to make sure is to contact the user and conduct an extensive investigation. A lot of time and money that has to be invested by bunq, and ultimately the user is probably not even very happy about it. That's the reason why in some cases bunq would rather say that they just don't support these kind of transactions at all, as there's really no way to do these and make everyone happy: legislator, shareholders and the user.
Does it mean crypto currencies are illegal in The Netherlands? No. But not everything can be easily accommodated for just because it's not illegal. Popularity would certainly help here, but despite your claims of them "getting more and more popular", growth has staggered since beginning of 2021 and I would personally say it's not really relevant to most people right now. If that ever changes, then sure bunq would probably also reconsider their procedures.