> Try mounting a Nikon lens on a Canon camera, for example.
You can simply buy the adapter from amazon or almost any shop. You've really chosen a bad example because companies have tried to patent their connectors to limit production of these kinds of adapters and the courts always rule the patents invalid. Locking people in for cash is a common illegal practice; it happens and take time to remediate, but is and should be illegal.
Yes, even if you want to use autofocus. I don't really like posting product links, but you can google it. Maybe at some point in your past you were personally not able to find such a product and make a purchase and that may have shaped your ideas of the situation. Also they keep making new connector types to keep this arms race going. As I said, the law takes a dim view of such shenanigans and certainly protects interoperability.
You can simply buy the adapter from amazon or almost any shop. You've really chosen a bad example because companies have tried to patent their connectors to limit production of these kinds of adapters and the courts always rule the patents invalid. Locking people in for cash is a common illegal practice; it happens and take time to remediate, but is and should be illegal.