Yes, I'm well aware of these restrictions. However, I don't think they impact whether or not you own the THING.
What GM, Ford, John Deere, Apple, etc. have said is that you can't tinker with their firmware that is installed on the device. None of them would argue that they can come repossess your tractor/car/mobile phone.
As others have pointed out, it is not currently illegal to jailbreak a phone, though that may change in the near future.
To make an analogy and play devil's advocate a little further, it would be hard to argue that I don't own my home. There is a deed to that effect recorded with the county, more than I can say for anything else I own. Yet:
* The state can take it from me in short order if I don't pay my taxes
* I have to get a permit from the city to make any substantial modifications to the property
* There is a neighborhood association that dictates property uses as well, which I have to comply with
* I couldn't just knock the house down and build a car repair garage -- city zoning ordinances restrict how I use my property.
And yet, I don't hear a big outcry that I don't truly own my house.