The fight between entrepreneurs and regulators over the so-called sharing economy is entering weird new territory.

With the arrival of Monkey Parking–an app that lets parking-space squatters auction their prized curb space to drivers weary of circling the block–startups are now making it possible to "share" things you don't own in the first place. The service has been available for several weeks in San Francisco–a notoriously difficult place to park–but that was long enough for city officials. San Francisco City Attorney Dennis Herrera has now demanded that Monkey Parking stop operating in the city. He has threatened to fine the Rome-based startup and two other companies with similar apps $2,500 per transaction, and he says anyone who uses the app could be hit with a $300 penalty.

"Monkey Parking's business model is wholly premised on illegal transactions," Herrera said, citing city laws against buying and selling public on-street parking. "People are free to rent out their own private driveways and garage spaces should they choose to do so. But we will not abide businesses that hold hostage on-street public parking spots for their own private profit."

Monkey Parking CEO and co-founder Paolo Dobrowolny told WIRED in an email that he couldn't comment specifically on the city's order while he consulted with lawyers. "As a general principle we believe that a new company providing value to people should be regulated and not banned," he said.

In the past, other online marketplaces such as Uber and Airbnb that faced similar crackdowns on their way to becoming multi-billion-dollar companies have offered variations on the argument that they simply act as facilitators connecting supply with demand. For those companies, however, supply generally consists of private property: someone's car or apartment. Monkey Parking, on the other hand, appears to be premised on the idea that it's okay to cordon off the commons for private gain as long as you get there first.

Parking in many cities truly sucks, a "pain point" toward which tech-minded entrepreneurs were bound to gravitate. Judging by the public outcry so far, however, spot-squatting isn't a solution that seems likely to withstand the political backlash. While the legality of Uber and Airbnb in many cities, including San Francisco, is ambiguous at best, both have thrived because the services they offer are incredibly popular. Their opponents are mainly the entrenched industries–taxis and hotels–that their new business models threaten to undermine. But politicians, especially in San Francisco, have been wary of alienating consumers and tarnishing their pro-innovation cred.

Messing with street parking, on the other hand, turns everyone behind the wheel into a potential enemy. Perhaps Monkey Parking will fight off the legal challenge in San Francisco long enough to show it really is a more efficient way to parcel out parking. But city officials can likely count on the traditional kindergarten definition of sharing to win popular support: Use something until you're done, then let somebody else have a turn.