having a nominal charge would probably lead to less abuse of the system, and any contribution to the upkeep/maintenance would be welcome, I'd imagine
On the other hand, no charges mean you can get rid of a lot of cruft: no tickets, no gates/turnstiles, no machines, no payments, no paperwork thereof, no ticket inspectors, etc etc. So in fact having 0 charge is unequivocally better than having a residual charge.
In other words: charge price = cost, or don't charge at all and get funded by public revenue.
What kind of abuse are you thinking of?