Honestly I think it is a joke that airlines can even enforce the way people purchase their tickets. They are unregulated in charging fares, why are customrs regulated in how they purchase?

I have gone somewhere on two RT tickets discarding the returns because w/o the Sat. stay, the ticket was 3 times the price. It was the company & travel agent who decided to do this (they also did back to backs for other people) and as far as I know the airlines never caught on. This was a few years ago and the detection methods may be better. Though if caught, I would defer blame to the company and/or travel agent since I did not even schedule or buy the ticket - I did not give a rat's a** how much the company paid for the ticket.

If you wanted to do back to back, I wonder if this plan would work (note I am not advocating doing it). Lets suppose you wanted to fly (for example) BOS-PIT on a Monday-Friday schedule for four weeks in a row. What if you did this:

AirlineA: 1st Monday BOS-PIT, return 4th Friday PIT-BOS.

AirlineB: 1st Friday PIT-BOS, return 2nd Monday BOS-PIT.

AirlineB: 2nd Friday PIT-BOS, return 3rd Monday BOS-PIT.

AirlineB: 3rd Friday PIT-BOS, return 4th Monday BOS-PIT.

In this schedule, AirlineA overlaps the three flights on AirlineB and every flight has a Sat. night stay. I wonder if they would catch this, or even be able to do anything about it. Again, I am not advocating it and have never tried it, but I wonder if it would work. Especially if you bought the first round trip on AirlineA then bought the subsequent flights on AirlineB AFTER flying out on the first flight. Then, how could they prove that you did not go somewhere long term then later decide that you needed to return on weekends?