This one is complicated. I understand that Charles reneged on an arrangement in Monza that could have seen Vettel challenge him for pole (and then the race). This is a big, big problem with these kind of arrangements. How does a team punish its own driver? They will (nearly) always find a reason not to do it. So, you can see why the temptation is there for the driver to simply pocket all the benefits and then ignore the obligations. They are internal team matters, not breaches of the rules of motorsport.
There is a rich history of team orders going wrong in F1. All racing decisions need to be context dependent, and "handing back the place" is very dependent on track position of the two team-mates but also the competitors.
There also should be a fair opportunity for Vettel to try to win once he was in second position, or was that part of the deal? Was Leclerc supposed to allow Vettel into second in exchange for then not being challenged at all? That would be very murky, given that no championship is at stake.