Imagine that Ford only allows maintenance at company owned stores, because they don't sell replacement parts to dealers, then they charge $500 for a fan belt, etc.
Wait, how is this different from any consumer device that has a warranty?
edit:
e.g. let's say my apple laptop breaks, even if I could fix it or get it fixed by a third party for cheap, I have to take it to the apple store to not risk losing my warranty completely.
Well, many states have laws specifically for that: a car manufacturer's warranty cannot be cancelled just because the owner or someone the owner paid worked on the car.
The manufacturer's warranty can only be voided if the manufacturer can prove that the warranty claim arose directly due to the owner (or someone they paid). So if the owner changes their own oil and the rear bumper falls off? Warranty.