Even worse, my Dad met us for brunch and overstayed his allotted time in one of these systems by 8 minutes. Would be ok if they billed him the same rate perhaps, but they "fined" him $80 for the "violation". All the while trying to look like the local police as much as possible. All these companies are scam artists.
Billing the actual parking time down to the minute is pretty trivial, so having to reserve a specific time means they are intentionally trying to defraud you by either a) making you pay for time you won't use, or b) fining you for not reserving enough time.