Yeah, I'd agree.

Nobody forced United to offer the FareLock, and they actually don't on a lot of fares, especially close-in or where inventory is tight. If they're going to take the money, they need to provide the service paid for, which is holding the reservation for 3 or 7 days. Doesn't matter if you have overlapping reservations -- as others have noted, the primary purpose of FareLock is to lock in a reservation and price when your plans are still unknown.

I would agree with a DOT complaint at the very least.