If I win Lotto tomorrow and buy a 350ER and fly 14 of my friends and family around for free, why would I be a 'safer' pilot than if one of them paid me $1 to cover $1 of the $15,657 direct costs of the flight?
Because that dollar changes the attitude and expectations of the passenger. A passenger who pays nothing understands that they are taking on personal risk. A passenger who pays a dollar, has now bought their ride, and expects full commercial protection.
There are are many times in law where a token payment changes the entire nature of the transaction. Peppercorn rents etc.