In many cities I think that this is a good idea, but in New York I worry about how effective this would be because of the outsized number of non-resident, non-tax-paying riders (tourists) who use the system.
That farebox revenue needs to cover operational costs for public infrastructure seems just crazy to me. The fire department doesn't need to make a profit on putting out fires. Public parks don't need to make money by charging an admission fee. It's just something we pay for for public good. I'm pretty sure that public transportation fares are something nobody has ever really thought about; the networks used to be run by private companies, and when they were taken over by the government (which put them out of businesses by building highways) nobody ever thought to get rid of the fares. It's certainly time to think about it.
They're literally carrying money into the local economy, yet one should worry about them getting a free ride on the subway along with everyone else?
A taxpayer may need months to generate the same amount of wealth a tourist leaves behind in a single week.
Making public transport free/cheap for only tourists would make more sense. In fact some places do just that (usually bundled with discounts/tickets to popular attractions).