The problem with that is it creates a perverse incentive for stations to not deliver power as quickly as possible.
If you're paying per kWh, then it's in the station's interest to charge you up as fast as possible so that you can move on and another customer can use the charger.
If you're paying per minute, then it's in the station's interest to deliver power as slowly as the customer will accept, so that you have to charge as long as possible (and therefore they don't have to pay for as much electricity).
At least in my area, I find that slow or broken chargers are much more common than cars that are done charging hogging the spots. I've only once had to wait at a quick charge station (and even in that case, I arrived first, but let another person in more of a hurry cut in front of me since I wasn't in a particular rush).