Repeated scamming SDC is a clear violation of Rule 5(h) of the COC. It is a fictitious booking which the passenger does not intend to fly. If UA cared about it, UA could: 1) limit SDC to 1x per segment or or using its monitoring capability to locate people doing this and voiding there tickets under the COC.
But, UA doesn't. This suggests that UA makes money off the practice. Few likely have the time to engage in this stuff in the first place. Of those who do, some louse up and the flight departs and the ticket has $0 value. Or there is no availability and the pax can't SDC at all. Or there is no space in the fare bucket and the pax has to then pay $200 + substantial fare difference.
So, UA likely figures out that the House wins on this one, so it lets a few slide.