sigma - Why don't Canon and Nikon lock out third-party lens makers?

Odd question but looking at the market, I see Tamron/Sigma making lens for first-party companies such as Canon, Nikon and Sony.

For manual lens such as Zeiss ( for Canon/Nikon) I see that they aren't competing with Canon/Nikon directly since all the lens provides is EXIF/IPTC data so that wouldn't impact the native lens that they sell directly. (Also, the price range is different (Otus) or they are made for video (Milvus)).

Is there a reason why Canon and Nikon allow third party makers to make lenses that provide similar specs to first-party lens such are AF, IS and aperture control when they sell for less? It seems that Canon/Nikon could block Sigma and other 3d party lens makers just with firmware updates, not that everyone would be happy about it.

It seems that Tamron/Tokina does license Nikon lenses.

Answer

Is there a reason why Canon and Nikon allow third party makers to make lens that provide similar specs to first-party lens such are AF, IS and aperture control when they sell for less?

Controlling a product line that has the support of many third parties is much more profitable than trying to build and sell every product yourself. Canon and Nikon don't just sell cameras and lenses, they control platforms that enjoy the support of hundreds of other companies. It makes them market leaders and establishes them as the best choices for most consumers, who want to know that they cameras and lenses that they buy now will be supported in the marketplace for a long time to come. That means that they sell more cameras, which translates to selling more lenses and accessories. Every manufacturer wants to control the platform, but only a few get to do that.

Both Canon and Nikon lenses have excellent reputations, and neither has to fear competition from lower priced manufacturers. They may lose some sales at the low end to lower priced manufacturers like Sigma and Tamron, and at the high end to companies like Zeiss, but having the support of those companies makes their entire product line that much more attractive and established.