That's the way Nissan does it, but there's no reason GM couldn't do it differently. They could require that the Chargers be available 24/7 in an accessible part of the lot, and if they are owned/managed centrally (eg by OnStar), corporate could set access and charging policies, too.
The dealer would have to agree to place the facilities, of course, but there are all sorts of carrots and sticks GM has to persuade dealers to do things like this.
Presumably Nissan has the same carrots and sticks. EVs will eventually bankrupt most dealers due to the reduced amount of service required, so they aren't going to do anything more than the minimum.