It sucks that Rivian isn't opening up their DCFC network but it ensures that their customers get a spot when they need it. Maybe they'll open their network after DCFC stations are ubiquitous enough that they have nothing to worry about.
That is NOT usually the problem.
For an example, Tesla SuperChargers can only be used with Telsa. But if another car is designed to use such, Elon musk said they would be open to other brands of EV's. But so far, nobody has designed a car to use them other than Tesla.
It can be more tricky than you expect. For an example, notice there is not even a single button on a Telsa Charge Station (other than the one on the handle that will open a Telsa charge door via wireless) . No way to make a payment or anything. When I plug in, it reads my VIN number and sends a charge to my Tesla account. So all I do is plug in and walk away. Almost too easy.
And DC voltages have to be in the program for different makes of vehicles. Battery voltages and such can be a lot different.
This is not an issue with AC charging because the input is very standard. And they can do whatever is necessary with each make of vehicle for its AC charger DC output.
Too many variables with charging with DC when away from a "standard" such as CCS (which Tesla cannot even use). The only real standard is J-1772, Can't find an EV that cannot use J-1772.
But on DC, we have a lot of different standards and that may never change. So we have CCS, CHAdeMO, Telsa Super chargers, Rivian and perhaps even more. But all of those vehicles can use J-1772 AC charging in one way or another.
-Don- Reno, NV