Because presumably, your tax money is already paying for it.
Free use of government property is a great thing when it's a public good, but parking is not one of those situations.
If tax money built the parking spot and tax money maintains the parking spot, then why should the government get extra income from the parking spot? Is my tax burden alleviated when I pay to use this parking spot?
It's not a matter of alleviating your tax burden, it's a matter of alleviating everyone else's tax burden so they're not subsidizing your car. You should pay for what you use unless there's a compelling reason for it to be subsidized, not the other way around. Roads are essential for freedom of movement and in any case are too difficult to toll on a massive scale, so it makes sense to subsidize them. Parking spaces are inessential and easy to charge for, so we should charge for them.
Paying to store your car is not a "tax" so it cannot result in being double-taxed. It's a standard payment in exchange for use of property.
Gonna need to provide some sources if you want to claim that public parking doesn't benefit society.
But parking at home? There's a straight line connection between the car owner who uses the space and the cost of the space. I shouldn't expect the government to pay for my parking any more than I should expect the government to pay for my house. In fact, paying for people's houses would be a lot more reasonable, since people need housing, but don't need parking.
I can't come up with any such counterarguments for public parking. It doesn't benefit non-drivers in any way, and the people it does benefit mostly don't need public assistance.
Edit: who pays majority of taxes for road use? It's car owners. You have cheap buses only because car owners paid majority of the bills for road network maintenance and development.
[1] https://en.wikipedia.org/wiki/Mass_transit_in_the_United_Sta... [2] https://taxfoundation.org/state-gasoline-tax-rates-2017/
Gas taxes don't cover all road expenses. I looked at Virginia's data (since that's where I live) and nearly as much road construction/maintenance money comes from normal sales taxes as comes from gas taxes. Another substantial chunk comes from the Federal government, and the Feds pay out substantially more than they receive from the Federal gas tax.
I don't expect private cars to ever disappear, but starting to line up the economic incentives to to disincentivize private car use makes a lot of sense if we're really serious about addressing global warming.
Besides, last time I was in SF I didn't see any free parking. It was all meters and expensive garages.
Free parking is rare in SF, which makes it all the more unusual that the people on this street had it for thirty years because they didn't pay their taxes for it.