I think idle fees will be the initial way Tesla will try to dissuade those who park at SC sites. First time, they get a warning. Second time, a modest fee will be assessed against their credit card that is on file, just for argument's sake, $25. Third time, there will be a base fee of $25, plus $2 per minute or fraction thereof. The clock will reset to zero each calendar quarter.
I do not think that Tesla should monitor the amount of time our cars are plugged in and charging because there are too many variables that work against a speedy charge: cold weather, cold battery, reduced charging rates, shared stalls and the necessity for a range charge.
Of course, nothing would prevent someone from charging for 45 minutes and then unplugging but leaving the car unattended. I have no clue as to how to monitor that situation.