Interestingly enough, I found that you can fly Omaha to Seattle on a one-way flight, but not Seattle to Omaha. Even booking a round-trip ticket, it gives you the notification when you try to pick a return date.
I can't imagine this will cause their traffic to increase to Seattle, since they only fly people to Seattle (so they get stuck there? Not a bad place to be stuck, I suppose...

)
So for those in Seattle, apparently if you want to fly to certain destinations it will work (Kansas City/MCI worked). I find it very strange, maybe they are not selling the tickets because the fares are too competitive for that route.
To sum up my findings, for Spring 2015 (March-April):
- OMA-SEA: Bookable
- SEA-OMA: Not bookable
- SEA-DEN: Bookable
- DEN-OMA: Bookable
- SEA-MCI: Bookable
- MCI-SEA: Bookable
Oh well, just another reason to use up my FF miles, give up my credit card and stop flying Frontier.

It used to be that they either flew someplace or not, with a consistent schedule, now it's very seasonal and they're doing weird things with the routes and whether you're "allowed" to book someplace or not... probably a matter of time before they remove the Seattle flights as well, since it's a cold weather to cold weather flight.