Because Tesla should be a subject of the NTSB investigation, they should _not_ be a party to it, any more than a suspected criminal should be a party to their criminal investigation. Tesla should have no discretion over what data they provide to the NTSB. Calling them a "party" sounds like an excuse to control the information flow, not narrowly tailored to a legitimate state interest.

Tesla has an ethical and fiduciary duty to carry out their own independent investigation, to the extent that it doesn't interfere with the NTSB's. These organizations do not have the same interests. They don't have to be adversaries, but it's inappropriate for them to be partners. The public is better served by multiple independent investigations.