The heat flux through the walls depends on the temperature of your apartment and your neighbor's apartments -- not on if your thermostat is on or off. So even if you leave your thermostat on at 19° you still benefit. And you have no obligation to turn your thermostat up.
The argument for theft would have to assume that there is a "right" temperature to set. Personally I would be sweating in a t-shirt if I had the heating set to 22C, which would be the only way to prevent the cooling in the example.
As others have pointed out, it's a happy coincidence that the equilibrium is at a point that GP is happy with. If it were higher they would have to actively cool their apartment which can be seen as a cost, sort of comparable to dumping trash on someone's property forcing them to pay to deal with it.
