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.