I don't think any reasonable person would think a Canon zoom would be priced similarly to a third-party zoom of the same specifications. By competitor, Craig is stating it would be so on a focal length, aperture, and feature standpoint. You buy third-party gear for the better price, and name brand gear for the quality (in most cases) at a higher cost. If one party does not have a piece of equipment with similar specifications that the other does (think Canon 200mm f/2L, 8-15mm Fisheye or Sigma 50-500mm, 12-24mm, 120-300mm), there is no competition in that segment.
Kyle, I get what you mean. but I feel that your last sentence, while generally accurate, leaves out the issue of price. not only does a piece of equipment need similar specifications, it needs to be somewhere in the same ballpark in terms of price. these days it doesn't feel like canon and sigma are even playing in the same time zone. I realize why this is a mutually beneficial arrangement from a business standpoint, as the consumer I can't help wishing for a more competitive landscape. I guess that's where Sigma is starting to go with new lenses like the 35 f/1.4; I can only hope that this trend continues. I'd love to see Sigma try their hand (as they have in the past) with high quality superteles with their new quality and design measures in place.