More like how your car can't require its own brand of oil, gas, air filters, light bulbs, etc. If Cisco starts selling its operating system on a CD and you make a clone that can run it, it's fair game. Or another company that makes specialized firmware can sell it so you can install it on your car, assuming it passes regulatory requirements for safety (I'd assume).
http://www.google.com/search?q=illegal+tying