For me, it comes down to this: they set the rules. All of them.

The airlines know mileage-runners and segment-runners exist. If they decide to put an end to the game, they can do so immediately. Minimum-stay requirements, maximum-segment limits. Some of these are already in place on certain routes/fares. (For example, there is a limit to how wacky a routing AA.com or United.com will book, isn't there?)

My take is that airlines should do more to embrace mileage-runners. (Disclaimer: I have never taken a pure mileage run in my life and doubt I ever will.) These are some of the most knowledgeable people about the product, and they are likely also some of the best brand loyalists. I'm talking beyond the normal level of "loyalty" instilled by FFP's in general: I'm talking about active, viral loyalists who are known in their various circles of influence as devoted fans of their airlines. People with the credibility to educate and convert additional customers.

Why Delta (or others) would seek to push these people away or discourage them is a mystery to me. It's not like an airline ever takes an incremental loss when a passenger completes a mileage run. AA's approach makes more sense to me.