when you pay to charge your car away from home presumably you’re doing it because you’ve run out of juice – so you’re not really buying electricity, you’re buying a way to get back on the road. this is a form of Value Based Pricing. the cost to provide it are somewhat irrelevant (as long as they’re covered). see https://en.wikipedia.org/wiki/Value-based_pricing
paying by time with “free charging” is really renting the space and reflects the opportunity cost of that space – if you’re filling up (generating revenue) someone else can’t.
the government “wants” companies to introduce PAYG at all charging points (https://www.autocar.co.uk/car-news/industry/electric-chargers-should-offer-card-payment-2020) and it does make sense – they will probably charge more to PAYG customers but given that most people will charge where they know they can charge, again, this is value-based. Remember we’re still in the early days of this infrastructure. doesnt it remind you of this: https://en.wikipedia.org/wiki/List_of_pre-nationalisation_UK_electric_power_companies