You see the problem is it works both ways. Canon implies a level of truth or of fact, to fiction just as it does in religion, but in the case of fiction it isn't there (notwithstanding the person who walks up to tv actors and seems to, or genuinely, believes that they are the characters they portray). You may fall on either side of the argument that the Doctor is half human, or that Hawkeye has siblings or a spouse, or that Solo is too old not to remember the old Jedi Order. But try to support your argument and all you can point to are works of fiction, not fact. Even endorsement by the property's creator(s) is no measure, because they change their minds, or lie, or forget, or don't care, or took the job over from someone else, or die, or any combination of the above. Sherlock Holmes fans argued (probably still do) about which of two candidates is the Baker Street house that Holmes and Watson lived in because it was fun ... and because they wanted to put a plaque on the correct house ... but screen property fans argue on the internet which stories "count" or don't "count" because I'm right and you're wrong end of discussion goddammit!!.
Once when presenting this argument in a fanfiction forum, I had it put to me that the usage of canon I appear to be employing doesn't match any dictionary definition of the word. I concluded that that's irrelevent, because the usage I seem to be employing is, dictionary definition or not, the usage employed by the fans I'm discussing: as if being factual and being imaginary were not binary conditions, as if there were degrees of being true or of being imaginary, and as if some works fall into some sort of hybrid category and some don't. Well, they are binary conditions, and they're conditions that describe the antithesis of each other. One of the earliest philosophical principles was that no thing can possess an attribute and that attribute's opposite.
When canon means what's "true" as it does in its usage in fiction fandoms (just like that, with the quotation marks), and fiction means what's imaginary, applying canon to bodies of fiction is an oxymoron - an oxymoron that, in my experience, divides fandoms bitterly, religiously, and to no meaningful purpose, when the purpose of fandoms is to bring people together.
Even granting the applicability of canon to bodies of fiction, what practical difference does a fan's identification of canon make? Really, except to piss off other fans, what's genuinely accomplished? For your opinion of what's canon to make any practical difference, you must be someone actively adding to it, whether in the series' production office or in a fanfiction forum. The storytellers get to say what canon is, by applying it, at least for the duration of the consumption of their story. But fans' opinions don't have any practical effect on it (except, perhaps, as purchase of tie-in merchandise may be affected) outside their own flamewars.
British tv writer and Doctor Who novelist and screenwriter Paul Cornell has an essay in his blog from February 2007 in which he says many of the same things I'd been saying for years. Not everything: in the context of arguing that Doctor Who isn't one of them, he contends that there are franchise/folklores for whom authorities (e.g., Joss Whedon for BVS) exist to say what's canon and what isn't. But he argues that not having canon is a strength (at least in Doctor Who's case), on which he is in agreement with me. (But he's wrong when he says no one ever picked up on the alternate-Dalek-history theory he co-authored in The Discontinuity Guide. I use it in my fanfiction.)
So I've stopped applying canon to bodies of fiction. I'm not the only one. Russell T. Davies, the current producer of DOCTOR WHO, is on record that the word is not used in his offices. And the punchline of the anecdote about the Baker Street argument is the Holmesian who turned his back on his fellows muttering, "A plaque on both your houses."
However, I made this policy change some seven years after the creation of this sprawling website, through which canon is shot through like tribbles in air ducts. Therefore I elected not to even try to excise the usage of canon from existing pages here, so you'll still encounter it, often. Ignore it. It's no longer true. It doesn't "count" any more. It's not canon; it never was, there isn't any. I say, a plaque on all your houses.