......So, they seem to get away with it - apparently still plenty of customers are willing to "gamble" in exchange for lower fares ...
From the Spirit website: "Spirit Airlines is the leading Ultra Low Cost Carrier in the United States"
Ultra. Low. Cost. Not "full service". You pays yer money, you takes yer chances.
When I was travelling a lot for business, the main reason I ensured I built status with my regular full service airline was not for the lounge access or the shiny card , I did it because I wanted to be higher up the queue when something went wrong. Could I have flown cheaper? Absolutely. ... and I would have (just for example) been stranded in SFO when an aircraft went tech and I needed to be in NYC the next morning, or stuck in HKG overnight due to a late connection , instead of being put on an overnight flight to SYD.