well I submit that if they were to charge the fee for tickets bought at the airport then they would be in trouble. Simply, not selling tickets that way is not a violation as I see it.

A few weeks back at DEN I witnessed only 1 agent working the agent assistance line. Which is pretty surprising considering the number of flights.

I'm not surprised by that, realistically most people should be going over to the bag check line. What is shocking is how some of these F9 flyers just throw money away. When I have been in that line to buy tickets I heard one of their employees tell the person behind me it would cost them $15 per bag more to check it in the agent assist line instead of just punching in their code and getting it done in the bag check line. The guy said he didn't care and stood in the line. I looked behind me, he and his lady had 5 bags! Knowing how much they charge for 2nd and 3rd bags I guess what's another $75?

It is shocking. Though I am a little curious how Frontier handles it when, like I've seen here in Charlotte, none of the kiosks are working. I somehow bet they charge people like they had a choice.

I helped a passenger who was looking for a Frontier rep after the counter closed... I think they wanted to buy a ticket as they said they hadn't missed a flight or anything. I told them that United is handled by the same contractor (G2) so to go bug them. I think it yielded someone coming out of the office to at least speak to them.

Had an agent at DEN tell me this last month. The agents at the bag check (which by the way, was completely empty) weren't willing to sell me the ticket, said could only be done from the agent assistance line. I made a deal with the original agent to wait 45 minutes for the line to go down then she would help me. She agreed... even though the line had not really gone down, she did wave me over 45 minutes later. I got the impression this was really HER policy rather than the Frontier policy...

I prefer to buy tickets at the help counter inside Concourse A if the line is short, we'll see if they have the same new 'rule' next time. There's usually only one person working that counter so it's often a crapshoot there.

I submit that they would be taxed the 7.5% FAA excise tax they're avoiding by charging the CIC.
Here's a nice GAO report related to the excise tax: https://www.gao.gov/assets/gao-10-785.pdf

The station staff here at DCA have tried to say that they cannot sell tickets, period, per the manager's policy. Or they've tried to charge me the $25 agent assistance fee.
I think either a class complaint to the DOT, some class action, informing the IRS, or reaching out to your Senator (since they're currently reauthorizing the FAA and the tax provisions) could be helpful.

For not selling at the counter: From what I understand, for them to impose the 'carrier interface charge' for the site, they need to offer some sales channel that doesn't include this fee. Otherwise, it would be classified as an abusive fee. (Which I already believe it is.) Therefore, they are obligated to sell at the counter without this charge. The practice of claiming they don't sell at the counter is also abusive. If it were me, I would file a complaint with the DOT.

I filed a complaint with the DOT, my local US representative whose territory includes the airport area, and with one of my Senators. I looked into FAA but it's not really their concern. The tax angle is a good one that I didn't mention in my letters, I'll keep that one ready for a follow up.

Waiving the CIC for purchases at the airport has nothing to do with whether or not the charge is abusive (which isn't a relevent regulatory term as far as i am aware). It's simply tax avoidance. Only air fare and mandatory fees are subject to the federal excise tax so structuring a significant part of the ticket price as a nominally optional fee saves ULCCs (and theoretically their passengers, too) a significant amount of tax liability.

I didn't have any problems buying at den today

Frontier would lose its ability to charge the online carrier service fee if it does not have a way to purchase tickets without the fee under DOT rules. So the current practice of telling customers they can't sell tickets unless there are no people in line will shut down the service fee.

Right. Frontier management should clearly instruct their airport staff not to discriminate against customers on line who want to buy tickets. They will kill the golden tax goose. I could easily envision a very large fine -- probably equal to the tax -- if the DOT investigates Frontier over this practice and finds they are taking steps not to actually sell these tickets.

I would hope this happens and agree with the thinking but airlines have lots of leeway. As I stated in my OP, the lady threw out the it's a new policy line and then her supervisor refuted it without doing anything about it. Real easy to say to DOT hey we were slammed at the moment but of course we'll sell tickets when we aren't busy. Which is almost never. I'd be fine if the airline told me we sell them on these days between these times but of course they don't even want that to happen. Their goal is to make it almost impossible to overcome their fees but good luck to the DOT in proving that they never sell them.

No, agent cannot deny any customer who stood in line by pulling such a stunt. That would clearly be in violation of the terms with DOL and would result in the elimination of the privilege of their online CIC fee. Doubt if customer stood ground that agent would not service him. Just pull out ur phone to record the transaction if agent presses and time how long it takes them fold like a lawn chair.