In addition to the above, the superchargers have no interface. They have technology that recognizes the car and that's how they bill you. Some older Tesla's came with unlimited lifetime charging included, so the system has to know who you are in order to bill you. That part of the communication between the car and the charger is proprietary. If Tesla does open them up to other vehicles, the adapter will likely be tied to the account.
This isn't entirely true. When Tesla developed their current plug, the J1772 standard was already established. When they built the Roadster, the standard was not yet in place, but mostly there. The Roadster's electronics meet the standard, but the connector is different. Tesla could have used the standard connector for the newer cars, but chose not to.