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.
Really?
So on a cost-sharing flight with all POB being pilots - the vast majority of the cost-sharing flights I've engaged in over the last few decades - we were all expecting "full commercial protection"?
And in the case of a flight in which only the PIC is a pilot, why is CASA allowing the other e.g. 5 POB who may be sharing the direct costs not to be afforded "full commercial protection"?