I suspect that the number of fatigue reports they received from the crews operating back would hopefully put a stop to that pretty quickly. The crew sitting in the cabin on the way out get no “rest”, and are just as tired arriving at the destination as the crew who operated out, more so in fact as the seats in the cabin are far less comfortable than on the flight deck, even on a 737. This tired crew then have to jump in the front and fly back. It maybe “legal” as they are technically only operating one sector thus extending the maximum flight duty period, but it certainly isn’t safe.
Whilst operating a single sector increased your available FDP under JAR rules, unfortunately EASA groups 1 and 2 sectors together with the same available FDP.