Why not? IIRK Musk has said that another company using the network would have to pick up its share of the cost of that network meaning that it would be required to pay its proportional share of capital costs (for expansion), maintenance costs and, of course, for the electricity that people with its brand of vehicle buy from the network. They are free to mark up any or all of these components as much as they want to IOW they could make their use of the network profitable. That's probably not a very smart thing to do. At least initially the SC network was, IMO, a marketing expense and I'd think a smart company, like Rivian, would recognize that and manage their participation at a loss, at least at first, just as Tesla did.