As for what is an "obvious" mileage run: this is not a court of law, and they needn't eliminate all reasonable doubt. They can very easily set up a quick formula (say, repeated short travels on ultra-low fares), and they don't have to justify it to anyone if some non-mileage runners get snared in the net.

As for UA not being against mileage runs: I never said they are. But I do think they would treat mileage runners differently as far as targeted bonus miles promotions and maybe some other informal perks. If I were a UA shareholder, I would prefer that targeted bonus miles promos go to the folks who buy a $1,500 walkup ORD-DEN ticket, not those who sit with their mileage calculators working out every possible cents-per-mile arbitrage opportunity.