The complaint isn't about a particular instance of poor to no inventory. It's about a widespread practice of same, arguably violating DOT's guidance about the level of availability that must be offered if a carrier wants to "advertise" a fare or redemption ("advertise" meaning telling people about it, not just buy TV ads and whatnot). And then when a consumer believes the carrier's statement and tries to redeem, the consumer finds it can't be done -- and backing up (to switch to some other redemption) purportedly requires paying a fee. I can definitely see the DOT being receptive.
Some folks might enjoy this
DOT guidance from 1994:
The Department’s deceptive practices prohibitions preclude airlines from imposing unreasonable capacity restrictions and/or unannounced blackout dates for the use of frequent flyer awards.To the extent airlines offer their frequent flyers awards for service to destinations, that traditionally are subject to high consumer demand, they must include in their promotional materials adequate disclosure that seats are limited—at times severely—and may not be available on every flight, if in fact that is the case.
...
If the number of seats that are made available at an advertised discount fare is limited and/or they are not available on every flight, the ad should specifically state either or both restrictions, as applicable.
See also
http://www.regulations.gov/#!documen...2014-0001-0012 (Southwest fined for no seats available) and
http://www.dot.gov/sites/dot.dev/fil...s/19941220.pdf (DOT letter to AA then-CEO Crandall re the need for "reasonable quantities for a reasonable period").