Operating a good wifi network at scale is usually quite costly. It is a good idea in some settings to be able to monetize outsized users. For example, your first 30 minutes on the airport wifi can be free, and if you have to be there for longer than that, you can pay a fee for the next hour.
How does a captive portal help with throttling or locking out users? Is the idea that the user could just recycle their MAC and be let in again, well the same thing applies for captive portals.
In general, someone needs to take a quick look it the development time and customer support involved really can be motivated from the extra earnings. The trend is that fewer and fewer bother, and just see complimentary wifi as a value add instead.
Airports, cruise ships, and other places where it does make economic sense are better served by real things like 802.11x and per-user QR codes.
That's not free, that's paying for it in a different way.
I do think the internet should be free, but how we sustain that is a pretty viable question. APs need to be installed, configured, and monitored. When spaces grow, APs usually need to be reconfigured or moved.
I know that airlines are fairly low margin businesses, based on what I've read from the recent bankruptcy stuff. I am curious about who owns airports and what their margins look like.