Ooh, I'd love to hear that one in court.

"Your Honor, that sign on the entrance to the parking lot discussing parking restrictions and enforcement thereof somehow implied that they're giving me this device and authorization to use it how I see fit".

I suppose if someone leaves their wallet on the hood of your car while they're tying their shoelace, they've also authorized you to use their credit cards, right?

If your car gets booted and you pick the lock to remove it can you now legally sell the boot?

They abandoned it.

the same way the dockless scooter companies abandoned their scooters?

Yes. You can buy dockless scooters from some police auctions.