I just wanted to chime in here that this policy is fairly common among US carriers, but many carriers in other countries will require you to purchase a new ticket at the prevailing fare if you have purchased the cheapest ticket. Qantas is one example that I can speak of from personal experience.

Running late for a flight from MEL-SYD that was a red-e fare, I wasn't too worried as I thought I could just hop on the next flight standby. Well, after a quick call from the cab to telesales, I soon learned that was most certainly not the case.

Luckily for me, traveling domestically in Australia is how it used to be here 10-15 years ago. Minimum checkin time was 15 minutes before departure, and curb to gate in 5 minutes! :-D