They didn't force them to technically. They voided the original and left them with a choice of finding alternative means of transportation or buying an inflated ticket. If you violate the TOS of your transit ticket they can terminate the ticket agreement. It's totally a scumbag move, but probably not a slam dunk lawsuit.

It would be like if you used a hack to get a cheaper uber and they refused to take you the rest of your ride until you paid without the hack. The hack may be totally legal, but within a private contract it constitutes a violation of your user agreement. Now you can walk, take the bus, hitchhike, rent a U-Haul, or whatever you want to continue the trip. But uber can say you aren't going anywhere in an uber until you comply with their TOS. Again, scumbag Mr. Burns capitalism, but legal and unlikely to be worth civil litigation.

I think it's a supply and demand (for pilots) sort of thing. There are less pilots than there are available flights. Pilots are pretty limited regarding their available flying hours (at least in the US):

An airline transport pilot can fly up to 8 hours per 24 hour period and up to 10 hours if a second pilot is aboard. Pilots are required to rest a minimum of 16 hours postflight. Some variances to these regulations exist depending on the company's operations specifications.

I agree about the cartel bit though, the prices for a lot of things are outrageous.

Did the airline detain him or did the security of the airport detain him. Two different companies in my mind. Does the airline have security and a security room in every airport? Opens up a shift load of extra liability.