It's like back in the dial up modem days with the Kflex .vs. the X2 protocol, even though Hayes was the so called standard..if I remember that far back correctly.
Tesla can sell to whomever they want. Other people are making AND selling an adapter already. I already have one and have been testing it. It'd be in Tesla's best interest to let anyone / everyone with CCS use their stations as they can charge them to do so but they don't HAVE to, no matter what some may think. I think the biggest hurdle right now would be them having to retrofit the adapter TO the fast chargers they already have deployed, but if one were to bring their own adapter, then it's just a matter of them saying, ok use your app and lets do this.
Many of the car manu's are selling / offering this adapter to their older customers to merge with Tesla super chargers. I think its only a matter of time before Tesla says, get the app, sign up, log in, get a good adapter and plug in and go.
Aaron