There is probably more then meets the eye here. The telecom regulator (OPTA) came into action after a complaint from a telco. It is not know what the complain is about but probably something about unfair competition ("we have to register as an ISP and the hotels get a free ride"). Currently OPTA is investigating if hotel wifi is a "public electronic communicationsnetwork". If they conclude hotel wifi falls into that definition then hotels (but also Starbucks and McDonalds) have to fulfill all obligations under the Dutch Telecommunications Act. And those are making the network ready for wiretapping and data retention. And that is not limited to responding to a wiretap warrant. They'll have to adjust their network so that they can execute the wiretap according to specs in the regulation. Those specs also require security measures for the wiretap equipment, screened personnel to handle warrants, etc. In the end hotels will conclude that this is costly and complicated. That is when the telco steps in (remember, they complained to the regulator). They can offer hotspots with all wiretap and data retention obligations already implemented. Profit!
Hotels can of course easily fix the problem - if open wifi turns out to fall within that definition in the law - by requiring a password for wifi access. After that it's not pubic wifi anymore.