They're not selling the spots, they're selling prior knowledge of their departure. They have no right or means to block or make exclusive the spots, so what's the problem?
The app itself invites users to 'request a parking spot' and asks 'how much would you pay?' The knowledge of imminent departure is just an instrumentality for gaining occupancy of the parking spot, to claim otherwise is disingenuous. There's a mentality among some hackers that 'the means justify the ends,' ie if you can find some definitional wiggle room on what you're doing it automatically validates the outcome. That works OK on computers, but the law is not a big computer system and does not operate on the same sort of binary logic as computers do.
"It shall be unlawful for any person, firm or corporation to enter into a lease, rental agreement or contract of any kind, written or oral, with or without compensation, for the use of any street or sidewalk."
According to the article it's plain illegal.That's the problem.