This is egregious.
The way it is "disclosed" is in the breakout under the taxes & fees. You get a listing like:
Passenger Civil Security Fee 5.00
Passenger Facility Fee 9.00
Passenger Usage Fee 9.80
Segment Fee 7.20
By the name and the placement they're trying to make it look like a government-imposed tax, when it's not.
This mimics a "worst practice" of certain LCCs globally like Tiger Airways that similarly hide airline-imposed fees in with taxes. The worst of the worst don't even provide a breakout. This falls somewhere between Air Asia (bundled, but better truth in labeling) and Tiger (no breakout at all).
I think we all realize that the fee is a money grab by Spirit, and maybe they can't sell tickets for $8 or $9 or whatever it is this week because too many people aren't paying the baggage/seat fees, their costs are up or any number of other reasons why they need more money to provide the service. That much is understandable (much as some might try to argue the straw man, anyway).
But the WAY they're going about this is patently deceptive and frankly it should be made illegal to "bundle" privately imposed fees with taxes. Doing so gives people a misimpression of how much the taxes really are (though the "real" taxes are still substantial), and also makes the fares look more and more like bait and switch.
The "honorable" thing for Spirit to do would be to raise the fare by $5 per segment. $8 sales become $13 sales, and so on. Instead they're taking the low road, using a very questionable practice that should be below the standards they should set for themselves. If companies do this sort of thing (like Priceline, or Tiger) and get away with it, it tends to drag down the standards of the whole industry (i.e. the dark side of competition).