Exactly.
To be technical, they were never not following canon. The gist of the argument is that (I assume) many people who have headcanons treat them almost as sacred as the true canon. They’re still following canon, but with a cherry on top.
And people don’t like their cherries being stolen.
I have another analogy: what would you think if Bethesda canonised the Dragonborn as being a Nord, when you built a whole backstory for your Argonian Dragonborn? Bethesda deliberately doesn’t canonise the race or gender of the protagonists in Elder Scrolls games precisely because of that.