That's not true and it's most definitely not a valid legal position to take in defense
It may fall afoul of the (terribly written, but still law) Section 1201 of the DMCA, if it's arguable that the circumvention provides access to computer software (and I think it is arguable).
I agree that this should be legal, but I'm not nearly as convinced that it is legal currently.
Recall that when CD copy protection was starting to be deployed, you could negate it with a black marker. The DMCA then made black markers illegal, because it bans "tools which may be used to circumvent copyright" as well as the "circumvention of copy protection measures". (Not that anyone tried to enforce it, but the letter of the law was pretty clear that black markers were then illegal to possess)
The DMCA should be amended or revoked to make these shenanigans no longer enforceable.
No court would ever come to the conclusion that something as mundane as black markers are illegal to possess.
The DMCA is a problem because putting copy protection on (for example) heated seats is a similar level of silliness to outlawing black markers.
That is wrong. It's legal to possess a hammer but illegal to bash someone on the head with one. It was legal to possess a black marker and arguably illegal to use it to circumvent copy protection.
(registered security researcher for my rooted Teslas)
Copyright has zero application here unless you redistribute Tesla's code or binaries. Reverse-engineering is a legal practice (in US law, at least).
Reverse engineering to utilize software you didn't pay for and don't have a license or some other legal authority to use is unlikely to pass muster legally, versus for interoperability purposes.
https://www.eff.org/issues/coders/reverse-engineering-faq (Control-F "What Exceptions Does DMCA Section 1201 Have To Allow Reverse Engineering?")
https://www.theverge.com/2022/7/12/23204950/bmw-subscription...
Only for the kinds of “use” that involve the exclusive rights of the copyright owner (“use” generally does not), and even then not always, because there are exceptions to copyright.
If you sell me a book, I'm not allowed to copy that book and sell those copies. I am allowed to chop it up and and repaste my own copy however I see fit.
If I'm wrong and current copyright law does not permit this kind of thing, then current copyright law is bad and should be changed.
[1] https://www.law.cornell.edu/uscode/text/17/1201 (17 U.S. Code § 1201 - Circumvention of copyright protection systems)
[2] https://www.law.cornell.edu/definitions/uscode.php?width=840... (circumvent a technological measure (3) As used in this subsection— (A) to “circumvent a technological measure” means to descramble a scrambled work, to decrypt an encrypted work, or otherwise to avoid, bypass, remove, deactivate, or impair a technological measure, without the authority of the copyright owner; and (B) a technological measure “effectively controls access to a work” if the measure, in the ordinary course of its operation, requires the application of information, or a process or a treatment, with the authority of the copyright owner, to gain access to the work.)
As for "they didn't pay for the license"....no....you gave it to them without asking them to sign a license agreement. They paid you money, and you gave them a product. How they use it is up to them.