It is probably out of complete ignorance, and nothing else.
If you want a great example of how deep this ignorance goes, look at shutterbug magazine from last year. One edition had an article discussing backwards compatibility, and barely mentioned pentax. I wrote in and criticized them about the lack of mention about pentax and they published my letter later in the year. The in the summer they had a letter asking help in using screw mounts, and said that there were adaptors for canon and nikon but none of the functions would work, even metering.
I wrote in again, (they didn't print this one) and really tore into them and the lack of technical oversight, for not mentioning pentac and the functionality with older lenses, even though there was an article in the same edition discussing that very point. OOPS!
In general, even the photo press is ignorant of what pentax cameras can do.
What I find in reading articles these days is that they use canon and nikon (and photoshop) as adjectives describing what they do. If it is not one of these brands, it simpy does not exist.
It goes as far as an article about recropping a shot, saying that to do this you needed to open the image in lightroom 3 or photoshop 5, then crop. Why not just say
any photo editor?
I think they get paid based upon thenumber of times they mention a brand in the text. (but that's just me, and I seem to have hijacked the thread a little - sorry)