Update-2:30pm: Most of the $7 tickets are already sold out!
The fares actually are bookable round-trip at $7+ each way.
I tried out a RT from ISP-FLL for 05/07-05/15 and it priced out with $7 tickets both ways.
Warning: Spirit sneaks in fees ($12 for insurance and $9 for their travel club) Be sure to uncheck those boxes on the “Confirm Flights And Purchase” page!
The total for ISP-FLL including all taxes priced out at just $35 RT for me!