Should a customer pay as much for a 10 Mb/s internet service as a customer who has 100 Mb/s service? Should a person pay as much for two-course combo meal as someone who bought a three-course combo meal?
Those answers may be both yes, especially if either is blocking a higher paying customer from consuming, as is the case with chargers.
There's two issues: Each car's ability to receive charge AND if a car is already there, that charger is occupied, so others can't use it.
It's easy to develop a model where the former can be solved (per kwh, or a proxy). The latter is the issue. From a provider perspective, you get more income the more customers are using your device. However, if you setup a rate structure for a slow charging vehicle to be interested, then your ONLY port is blocked while the slow charger is plugged in and you are prevented from getting payment from a fast charging vehicle. If my vehicle can only take 100 and the charger can give out 200, which other cars can take, the provider would like to sell 200 to everyone continuously. So, if they were to let me charge my 100, they are not selling 200 to someone else in the meantime. [I intentionally used no units.]
Selling 100 is more income than selling 0 (good), but less income than selling 200 (bad). Some providers will set their price low to bring in more customers (potentially creating lines) and some will set their price higher to bring in fewer customers, but whose average transaction is higher. If demand exceeds supply, prices will rise. Basic economics.
Think of it like an airplane that has only one seat. That one seat will be priced more like 1st class than steerage.
I tried hard to come up examples from everyday life where "more important" people get bumping privileges over regular people and, other than airlines, I couldn't. We wait in line at gas stations, restaurants, grocery stores, etc. We hope the person in front of us, who's causing us to wait, moves along quickly, but there's nothing much we can do about it. All of that happens independent of how big each person's transaction is. True, many locations have multiple registers, like a 10 item or less line, but that would mean the solution is more chargers.
Lots of people will accept waiting an hour or more for a table at a fancy restaurant, while others will go somewhere else, likely to a less fancy restaurant. Some people will wait to get gas at Costco in order to save money, others won't. If you want the "fancy experience" of charging DC and are willing to wait in line, that means they're not charging enough. Conversely, if the charger is sitting there empty, they're charging too much. If you have to wait an hour to DC charge for an hour, you could have gotten 60 miles on board at an AC charger in that time.
So, I can understand why charging stations cost is based on how much they're capable of providing, even if some vehicles can't take the charge that fast. That doesn't mean I like it.
The Uber demand pricing model is spot on, in that, it attempts to rebalance supply in times of high demand, but look at the large amount of complaints that it was unfair, predatory, price gouging, etc.
