It is fair because people know what parking meters are and that there are consequences for parking in the spot without putting in all the money. If they didn't charge money for those spots in big cities, someone might park their car there all day and prevent others from using it.
I don't think this would be fair because people have unexpected issues that can come up last minute when trying to get back to their vehicles that have been parked. I have had personal experience with this while at a university and had to pay to park for specific amount of time. There have been instances where I had to stay after class and then when I went back out to my car, my meter time had expired and I had to pay a big fine. It just doesn't teach anything, it is a tool for revenue for the states and schools.