Public transit is subsidized in a lot of the world. Subsidized is good. Free though.. free is a problem. A good or service that someone doesn't have to pay for becomes valueless to them, despite it still costing someone (sponsor, owner)/something (corp, gov).

Examples:

A free Christmas party, where the cost of the convention center/catered dinner/wine on the table/entertainment was about $110/pp. Those costs were all paid in advance (you might be able to exceed catering, wine is probably fine but at a different rate.. but otherwise you're stuck with the bill/contract), but when tickets were free the no-show rate ("didn't cost me anything") was about 30%. We switched to $20/ticket, and yes there were grumbles about the company being cheap, but the no-shows fell to around 8%.

There's free sections of transit in our city (not the whole system). Most of the disruption and police call-outs happen in this section.. open drug use, screaming at/threatening other passengers or making them feel unsafe, making a home, thefts & pickpockets (does a thief buy a ticket? Perhaps if they spend 6 hours a day "working" and don't need the hassle of being checked for a ticket), even violent crime (stabbings, beatings, one gun situation). An unfortunate side effect of that is we have one of the worst riderships in part because people don't feel safe on transit, and when discussions of subsidies/improvement/expansion come up ridership is basically the only KPI. ("Oh only 12% of the city uses it, let's not further invest").

Transit is very heavily subsidized by taxes. In the US, the least money losing system is BART which covers 70%+ of it's operating (but not capital) expenses through fares.