Technically yes, they must pay. If they are there for a short while like an hour or so nobody would mind, depending on how dickish the owner of the property is.

For more long term parking say a couple days, it's liable to get towed.

I was working as a security guard in a warehouse type building, and there was this one car with a broken windshield that belonged to one of the employees. This car had been parked in the lot for about a month until the building owner had it towed (because he was a prick). Even though parking was free, and it wasn't a restricted area. Legally speaking it was within his right to tow that car, the employee ended up paying about 3-400 bucks to the tow company.