If they don't care why we fly, why would they be trying to prevent people flying for business from getting cheap rates? If they don't care who buys it, that is?

I think we broadly agree here, I'm merely using labels to make the point simpler. Irrespective of who is flying or why, it is in AA's interest to get the maximum revenue from each passenger and each seat. If AA can drive up fares for people doing same-day turns (for whatever underlying reason) without dampening profitability of the routes or passengers, they are incented to do so. Same for midweek overnights, whether the individual passengers in question are business travellers or flying to Los Angeles to watch a Lakers game.

One thing I would posit is that mileage runners are not as insignificant at the end of the year when a DEQM promo is going on. Just based on my work I have several friends and colleagues chasing status this year (on all 3 major alliances), and many of them are throwing in some same-day turns or unnecessary trips to ensure they make their status. With the exception of me, none of them call it "mileage running" or even realise that there are places where such things are widely discussed, they are simply dealing with end-of-year realities and trying to maximise their own benefits. It would be silly for my friend Kevin to end the year at 92,000 EQMs on UA when he lives near SFO--he knows that, so he's putting together a quick overnight somewhere cheap to top up. For UA, Kevin is a captive customer on this trip, he will pay whatever the price is to ensure he makes 1K, so it's in UA's interest to make it a bit more expensive for him while still filling those seats and ensuring maximum profitability. You're right that UA don't care who Kevin is or why he flies, but if they can get him to spend more without affecting his loyalty, shouldn't they?