So you are saying it is OK to cheat the airline out of their published fares because you don't like them, or consider them exhorbitant? Try that in a shop.
A shop could never get away with the type of pricing that airlines use. Cue the famous
if airlines sold paint analogy. (EDIT: beaten to the punch by Metro man above!)
I'm not endorsing the practice of H-C ticketing, and don't use it myself (although I confess to occasionally buying return tickets for one-way use). But I'm sure many travellers suspect that if an airline charges more for a trip A-B than it charges for A-B-C, then the price for the A-B trip is artificially inflated, and the airline is therefore cheating them. In such circumstances, they would see H-C ticketing not as cheating but as a morally justifiable way of righting the initial wrong.
But coming back to United's dilemma - cracking down on H-C ticketing is likely to draw attention to the practice and to increase its prevalence. The options for airlines are limited: it is one thing to blacklist 10 persistent abusers, but quite another to blacklist 10,000. The only real option is to adjust the pricing to make H-C unattractive. Whether this would be better or worse for consumers overall is hard to judge.