United probably decided that the PR headache wasn't worthwhile. According to Aviation Week the pax who bought the tickets were making a big stink, and there was a lot of media coverage.
Remember that United is trying to push superconsolidation through DOJ and is thus getting lots of flak over consumer issues. So it's not in their interest right now to get bad PR over something like this. Even though UA is probably correct that the law is on their side. I myself would not have booked one of the fares had I discovered it, figuring if it's too good to be true it's probably a mistake and thus a risk.
Need a new airline paint scheme? Better call Saul! (Bass that is)