I don't have a bunch of time to look up pages in books and quote them so I'm going to drop this.
Admittedly, I'm not 100% sure on this. I seem to recall the discussion vaguely from a few months back, but that's about it. GhostBear can feel free to jump in and correct me on this :)
I think the main problem is you're thinking about the wrong word. Everything published and approved by the writers/owners of BT is considered canon. Its part of the universe. So the novels are canon because they were written by writers under the direction of FASA for the line. On the other hand, fanfiction isn't canon. A few products that might have been semi-canon at one time, such as third party publications like fanzines, aren't necessarily canon now, due to issues of oversight, changes, etc. But officially published books are part of the canon.
What you're probably really discussing is if novels are, for the lack of a better phrase, Real Fact. If what happened in them is the Truth with a capital T. And for a while, it was that way. While sourcebooks were considered In Character publications, and thus limited by what the person in the universe knew, novels were considered to not have that filter. You were there watching the action, often from a third person omniscient view. You weren't reading a report about Victor skiing on Tharkad, you were watching Victor skiing on Tharkad, and thus you knew that the event occurred.
The change seems to be not that the novels are no longer canon, but that they aren't the final arbiter of what is the Truth of the universe. Just because you read something in the novel, doesn't mean it happened exactly that way.