What Spirit appears to be doing is emulating the fee structure of European
budget carriers such as RyanAir. The problem is that their fare prices are not in the same universe as the carriers they are copying.
The model works in Europe because people don't mind paying the extra fees when the fare itself is under $20 or so, which it often is on these European carriers if you buy in advance.
Here is a Ryanair example for a flight from Ireland to the UK, eight days from now (not a holiday in these countries)
Fare: 12.99 EUR
Credit card purchase surcharge: 6.00 EUR
1 checked bag (up to 45 lbs): 25.00 EUR
Seat selection: 5.00 EUR
Confirmation text message with flight info: 1.00 EUR
TOTAL: 49.99 EUR ($67.60 at today's exchange rate)
$67 is still a pretty good price for this flight, so people don't complain much because they know they are going with an airline that is really bare bones, and if you don't want to pay the add'l fees there are ways to avoid them (including buying a branded gift card from the airline to avoid the CC charges.)
Here are all of their possible fees, and the blurb on how to avoid paying them:
Admin Feeis avoidable by paying with Ryanairs recommended Mastercard Prepaid card.
Priority Boarding Fee - is avoidable by simply not selecting this service.
Reserved Seating Fee is avoidable by simply not selecting this service.
Airport Boarding Card Re-issue Fee is avoidable by simply presenting your online boarding pass at your departure airport.
Checked Baggage Fee - is avoidable by simply travelling with no checked baggage.
Excess Baggage Fee is avoidable by simply complying with Ryanairs agreed baggage weight limitations.
Online Check-In Fee is avoidable by simply purchasing the lowest advertised promotional fare that includes the online check-in fee free of charge