Arguably it should be both. Sitting and occupying a DC fast charging booth, especially once you're not charging at the full rate, represents an opportunity cost since someone else could be using it.

I used a Tesla charger (as a non-Tesla driver) recently. I think their pricing model is pretty good: pay per kWh (varies between peak and off-peak), and if the station is busy they can impose a "congestion charge" for anyone occupying a charger and not charging, or charging above 80% when it's not necessary for their journey (presumably only works for Teslas where the satnav knows about your journey and charge locations).

> especially once you're not charging at the full rate

I don't think you want that as no car will be able to charge at 250kW for very long for example.

And I believe we're starting to see even higher peak charging rates. As always, there's no simple answers