These offers happen every year. The problem I see is that United doesn't have its act together enough to send the offer out to everyone. It is not a good offer. Before I would even consider most of these offers I would have flown to some nice place for an enjoyable lunch or a quick weekend trip. For every hundred to ten thousand offers United makes someone will accept and United will make >90% profit.
If United were to make an offer which was less than the amount spent on flying, could you imagine the uproar from those who scheduled an extra trip on United to make the cut: those who actually flew to make the cut?