Fare-charging for public transit has significant frictional overhead. I think in Luxembourg they just made it all free and it didn't cost much money because they didn't need to spend anything on collecting fares. The D-Ticket in Germany too: in some cities, almost everyone has a D-Ticket so the frequency of ticket checks was drastically reduced.
Another counterpoint: if the bus isn't overloaded, taking an additional passenger costs next to nothing, while delivering significant value to the passenger. Don't we want to create as much value as possible?
You can see that a long-distance train has doors only at the end of carriages, and stops for several minutes, but a subway has doors about every 10 seats, and stops for 20 seconds.
Another counterpoint: if the bus isn't overloaded, taking an additional passenger costs next to nothing, while delivering significant value to the passenger. Don't we want to create as much value as possible?