The word "canon" is also an interesting choice for this concept. It references that the church - sticking to the most popular example - "decided" what was canon when christianity was forming, and so can authors about their works. Cynicism aside, I don't think religious leaders understood canonicity as their favored interpretation, but as actual truth, which is obviously not the case with modern media.
It’s not like fanfics or theories are illegal.
People always have their “headcanons”
edit: using an alt bc I use antiprocrast on may username and wanted to answer. hey, it's a saturday!
If it's the same author writing these pieces of fiction, then speaking by definition, the author's opinion is more special by being the creator of those works and therefore can create fiction that reinforces their headcanon (which is why it's called canon). So I think calling the author's opinion less special is wrong for the author's tie to the work will always be more special than the consumer by virtue of being the creator.
Official canon, at that point, is just a marketing tool. If you as the reader don’t like it, don’t engage with it.
The only real restriction placed upon readers is that they can’t sell their alternative stories under the franchise name, which seems fair to me. It’d cause too much market confusion otherwise.