Full Self Driving actually counts as deferred revenue to Tesla, and is reported as a liability on the balance sheet. They cant realize that income until the feature is delivered.
Don't underestimate power of accounting, mixed with loose ethics.
https://www.tesmanian.com/blogs/tesmanian-blog/tesla-autopil...
https://loupventures.com/teslas-software-advantage-is-clear-....
if it is a liability on their books, then i would agree with OP that it is an interest free loan.