It's in the public's response. If they understand that the carry-on fee is an add-on to the ticket fare, Spirit loses.
If the public is too stupid (remember: the average IQ is only 100), Spirit wins.
Spirit has now given me a means to reject them as a carrier when they show up as the lowest fare in my corporate booking system.
What if the tickets cost $15 less, now they they collect $20 for an overhead carry-on? Will you still pay more for a ticket on another airline, just to spite them for giving you the option of paying less?
If they charge the same for the under-seat option as others do for the overhead option, they will lose out, but I doubt that is their plan, and if it is, they will lose and it’s still none of Shumer’s or LaHood’s business.