Adding a 1-night minimum stay to fare requirements would allow airlines to shut down MR's without inconveniencing most other customers. At least, I don't see much of a population of fliers who really do need to fly to a far away destination and then immediately turn around. (Short distances, on the other hand, would be something else... SFO-LAX is a perfectly plausible business day trip, and so a 1-day SFO-LAX itinerary shouldn't be priced higher; but it would make a lousy MR for most folks anyway.)

Since such requirements haven't been instituted, I would assume like others have suggested on this thread that mileage runners don't exist in sufficient numbers to worry revenue management folks at most airlines.

P.S. The 1-night minimum stay requirement would have to be a little bit more complicated to take into account the possible incorporation of red-eye flights into a mileage run. But it shouldn't be hard to make the fare requirement be the passage of 1 night between the last outbound segment and the first return segment....