The whole buy-at-the-airport bit is the "TicketMaster model" where they act like you should pay EXTRA to buy over the internet rather than having to go to a physical location like an airport or box office. When both for Frontier and TicketMaster they're just not set up for it anyway, and it would be more expensive to hire employees, etc., if they were. It's cheaper for the companies to sell online, but rather than charging u less they look for an excuse to charge you more.

Both Frontier and Spirit Try to use sneaky mandatory fees (none worse than the CIC or "passenger usage fee") to try to build a base amount not subject to sales and that they hope to shield from fare competition. Legislatively they also want to go back to having all that HIDDEN, so that they could advertise a $0 fare and it could total out to $50 even before "extras" like bags. It would become one giant bait-and-switch, but you've got a long history all over the travel industry of advertising "bait" prices that don't reflect what the consumer actually pays.