Interesting topic, and great discussion. Very happy to see educational, constructive debate here and no "flaming".

I'm a pretty moral person, I guess (even though I do eat a grape at the grocery store to make sure they're not sour ). I'd probably call the airline and report the extra miles if they showed up in my account and shouldn't have. Hopefully, the airline would let me keep the miles, but part of the deal with calling and reporting an overage is that they may take the miles away (especially if it's a big error).

I don't know that it's the same as simply finding a $20 on the sidewalk, since you have no way to determine who the money on the sidewalk actually belongs to. I think it would be more like actually seeing a $20 bill fall out of someone's pocket onto the sidewalk and then taking it anyway, even though you know who it belongs to. Not illegal, just not the best choice, ethically speaking. Now if you offer the $20 bill back to the person and they let you keep it, it becomes a gift and it's yours to enjoy. Same with FF miles.

As far as the issue of receiving mileage for a ticket bought on one airline but re-routed to another - this is a different issue, because this is a flagrant breaking of the rules and terms of the frequent flyer programs (which you agreed to abide by when you enrolled in the program). You may not like the rules, or think they're fair, but you are still obligated to abide by them. It's just like going 5 miles over the speed limit on the interstate - seemingly harmless, but still against the law.

Basically when it comes down to it, doing just what the law requires may keep you out of trouble or jail, but it doesn't necessarily mean you are person with strong integrity. Also, since what is "moral" is a matter of opinion, not everyone will agree on exactly what morality is. It's been interesting to see everyone's opinions on this thread - keep up the interesting discussion!