Yeah, and Ford is going to sue Fiesta owners for fitting Regal tyres. Honestly, what's next ... the threat of legal action for the use of non-Canon lenses, unauthorised brands of CF cards and/or not "upgrading" within six months of the "replacement" model being announced.
Your analogy is fairly flawed. A more fitting car analogy would be Ford suing Fiesta owners for altering the software in the car's ECU.
The issue here isn't the users — Canon will never sue the users. The issue is with the developers of Magic Lantern. You enter some very tricky ground when you start reverse-engineering proprietary products that're covered by intellectual property laws. If you can prove that you've done a completely black-box reverse engineer (i.e., you did it completely on your own by observing the product(s) you have) then you're legally protected from lawsuits.
However, if there's even a whiff
of some inside information being used - be it a confidential informant from inside the company or unearthing some leaked document and using the information therein, you get into very hot water very quickly.
I imagine Canon are happy to let things slide a bit for "lesser" cameras and Magic Lantern etc. However, if
their stance on the EOS-1 is indeed what this thread is discussing, Canon's legal team will be looking for any possible way they can put pressure on the developers of Magic Lantern.
To be honest, I don't blame Magic Lantern's development team for keeping away from them. Sure, this all may just be smoke and mirrors from Canon, but from their point of view, it's not worth it. Someone posts a tip on their forum that helps development which later turns out to have been from an internal document? Bam - lawsuit.