You could apply this arguement to many things in Aviation.
Hypothetically my brother phones me up and says "Alan, lets do lunch in Jersey, my shout", and I fly us there. I pay for the plane at the time, then later on when we're back he bungs me a wad of cash to cover the costs......Illegal, yes, but I wonder how many people would do it .....
Of course, I could never condone such action, being an upstanding citizen, but the big difference in this case would be that I could trust my brother not to drop me in it........
I've had people offer to pay me (or rather the costs) to take various relatives on joy rides, including one guy who wanted me to take his dad on a birthday treat. I refuse becasue its not worth the hassle if it goes tits up. In the States however.......
EA