One thing that I've noticed with Canon DSLR camera is they will disable or limit autofocus when the lens' maximum aperture is narrower than certain values, which are specified in my answer to What apertures are required to enable autofocus, including cross-type or high-precision focusing, on Canon DSLR cameras? Note that Nikon cameras are also typically limited to f/5.6 except for models featuring the latest revision of the Multi-CAM 3500 AF system as in the D4 and D800.
As far as I am aware, Olympus and Pentax do not have these limitations, even if autofocus may be slow or unreliable beyond f/5.6. (Sony/Minolta requires at least f/6.3, with one exception for a 500mm f/8 autofocus mirror lens, and the Alpha 850 and 900 have a center AF point with high precision at f/2.8.) For example, on Pentax, 9 of the 11 points are cross-type and remain so at all apertures. The Pentax AF system has been reported to work at f/8 and beyond, though extremely slowly. Olympus Four Thirds cameras will continue to attempt to focus below f/5.6 as well. Why do Canon and Nikon choose to limit AF functionality instead of attempting to focus with reduced performance beyond the above cutoffs?
Answer
The auto focus systems are not capable of using the highest precision cross type focusing points if you do not use a large aperture lens. They aren't artificially limiting the precision, they are simply working within the constraints of the maximum aperture.
Canon puts these limits in to ensure reliable AF. If you don't believe that theory, add a piece of tape between your teleconverter and lens, on the top three left pins. This will trick your body into thinking that you don't have a teleconverter attached, and your lens is simply a 300/4.0 or whatever you have.
Furthermore, only the Canon 1-series cameras are capable of AF with a f/8 lens, albeit at the center point only. Usually this setup comes from combining a long lens with a extender. (Note: more recently the Canon 5D Mark III, 7D with firmware v.2.0 or later, and 7D Mark II can AF with the center point up to f/8. The Canon 5D Mark IV and 1D X Mark II can AF with multiple AF points with an f/8 lens. The Canon 1D X requires later firmware to AF an f/8 lens with the center point. As originally shipped it would only AF with f/5.6 and wider lenses.)
This really comes down to a issue of user experience. Sure, they could include ISO 204800 or 409600, but the quality would not make anyone happy. They are trying to keep the majority of users satisfied with what the camera can do, and most consumers are satisfied if AF doesn't hunt endlessly.
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