Because in the donated service (car pooling) its much less likely that the people participating are doing so as members of the public. Its like the difference

Because in the donated service (car pooling) its much less likely that the people participating are doing so as members of the public. Its like the difference between me showing a movie in my house to 15 friends And me renting out a movie theatre and opening the doors for everyone to come in. Even if the same 15 friends show up, they do it as members of the public.