Dan:
I suspect that Nikon/Canon (put any brand in here) doesn't have any control over knock-offs. Isn't that why they are called knock-offs?
I've read that one can buy a Rolex watch for much less than what they cost at a licensed Rolex dealership.
And I'll bet most people can't tell the difference.
I doubt that neither Canon nor Nikon sell their lens designs to third parties - but I also suspect that the competing third party designs have been reverse engineered, sometimes even with some internal changes in addition to the exterior cosmetics in order to avoid infringing on the original designs.
Glenn