RustyC

It's a revenue move...Wall Street is worried that people are catching on to the business model and adjusting behavior to avoid fees, so they have to keep throwing curves to please Wall Street. A real classic in that vein had to be when Spirit broke with industry standards and made 40 lbs. the checked-bag "standard". That meant that if you were connecting off a legacy carrier and had carried 50 lbs. there at no cost they'd still call it overweight and charge a fee ($25, I think it was) for that same bag check on Spirit IN ADDITION TO the checked-bag fee.

To their credit, most Spirit FAs don't want to be bag-policy enforcers or try to make people store bags under the seat if there's room overhead (which there almost always is unless the flight has a checked-bag "embargo"). Gate agents also don't like having to do the enforcement, which is why the soft, malleable bags are good if they can be squished into the space (e.g. rucksacks).