I’ve seen ads sometimes on airport WiFi, but rarely anything of consequence at hotels, other than a basic splash page promoting the hotel itself. The hotel also already has all your personal information from the booking, so I don’t see how WiFi could be any kind of significant driver of further data collection through a captive portal page.

The one thing they do have to do is make you accept terms and conditions, and that almost always seems to be the primary driving force on these pages.