Yep, Allegiant's "convenience fees" allow them to illegally (IMO) advertise fares that for all intents and purposes do not exist. By limiting the availability to airport ticket counters only, you make those low fares available for a matter of a few hours per week in some communities (not to mention the cost of getting to the airport, parking etc).
Yet the "fool the masses" business model seems to be working for them. People in places like Bismarck and Owensboro are probably glad to pay extra for the privilege of not having to run the ORD/ATL/DFW gauntlet to get to their vacation destination. The "convenience fee" is simply an "Oh well" for them.