On the Matter of Headcanons

So it's been a while since I've posted a blog post. Or story, for that matter. Do I have a story for you? Eh...I'll get back to you on that. But a blog post? Now, that I have.

We all have a headcanon about the show. Even if you say you don't and you just trust what the published materials say exclusively, that's still a headcanon. A boring headcanon, but a headcanon nonetheless. The only way to not have one is to be unaware of the show.
Now, a headcanon is great to have. It means you have your own little interpretation of the show's world and characters, and with that some great stories have come about, and I've taken some personal headcanon from such stories.
However. Something I consistently notice from some people is that they tend to view their headcanon as a matter of fact. In particular, this occurred on my friend ClanCrusher's story Winds of Change after he had Twilight learn changeling magic in one chapter. Now, I won't name names, but one commenter going by a name with the first initial of L took issue with this, claiming that Twilight couldn't learn Changeling magic because it's "not the same." Despite, y'know, it being stated within the context of the fic that magic is magic.
I will be the first to admit that I nearly jumped into the comments section that day despite my sworn oath to Clan not to. (I held back, of course, but I almost did it.) While he was utterly destroyed by another commenter, it presents an entirely different problem that I keep noticing.
See, despite whatever evidence one can find to support their own headcanon, unless it is explicitly confirmed in some place of the franchise, it is not absolute canon. L wasn't the only one to try this on the story, either.
When Clan posted Chapter 4 of that story, someone else weighed in on the subject of how mind control works, let's call him M, by claiming up and down that someone couldn't be aware that they were being influenced for it to work. Despite the presence of something that effectively translates to drugs. M even went on to state that he was drawing that opinion from other fan works. Why should those other pieces should be taken as an authority over this one? Do they possess some quality that gives them some inherent priority over this story?
Okay, look, readers, let me level with you here. It's perfectly fine to find something that violates your headcanon to be a bit jarring. Any time an assumption you've made for a while is challenged, it's particularly difficult to get as invested in a story as you could be if it didn't. That isn't what I'm criticizing here.
What I am criticizing is that some of these people to immediately jump in and start attacking the story for violating a headcanon the author would have had no way to predict unless it was a particularly pervasive headcanon, and even if that were the case, it's still a non-canon assumption that can be ignored. Seriously, all I hear when this comes along is something along the lines of "Oh, look, this person decided Octavia and Vinyl Scratch weren't associated in any way, shape, or form! Let's completely ignore whatever other merits this story has because of this one non-canon detail!"
Congratulations, you have a headcanon. Yes, it's good to have one. Everyone has one! But no one headcanon, no one interpretation, is inherently superior to another. No, not even if it's yours. Not yours, either. And especially not yours, I've seen your theories.