Most captive portals I run across are trying to verify that you are in fact a paying customer. Typically by making you enter your hotel room number and name or something.

Normally you would think it would just be easier to set a wifi password and skip the captive portal nonsense, but so many vendors harbor the fantasy that other people will pay for the wifi so they have to leave it open and put a "buy 24 hours of internet for only $30!" link in there.

I hate captive portals. They are such a nuisance, especially when you have your machine configured to always connect via VPN.