Can anyone explain to me, what possible logical sense does a 6 stay requirement make for the airlines?

If they are looking to fill seats by enticing customers with cheap fares (i.e. $600 r/t to SE Asia) how does the 6 days matter? My only guess is to discourage mileage runners, but then that's sort of counterproductive. If its to discourage business travelers from benefiting, why isn't a Saturday and/or Sunday stay requirement enough?

All opinions are welcome...

Gene