Words have an "agreed" on meaning. If you and I agree to call fried onion, steak, for us it is steak. Everyone else may disagree but among ourselves we are not wrong. For words like Science Fiction. Fantasy. or Technofantasy there is no complete agreement as to what they mean, so there is a lot of wiggle room.
Well that is somewhat the root of my point.
Words have an agreed upon meaning, which means the author and publisher alone should not be considered sole arbiter.
Now, if I were to read your statement as "in the case of a book's genre, I regard the author and publisher as the clear single authority and align my opinion of the word with theirs", I would still disagree as (from least to most important):
a) I don't agree with having single authorities on matters so trivial
b) While I'm not a hardcore death of the author type, I do generally take some cues from schools of thought that hold the author's intentions over the work should not be held as paramount to what the work is, as I find that increases the pleasure I take from many works.
c) Occasionally, some authors will provide opinions as to the genre they wrote in that lie outside common and dictionary usage by orders of such sufficient magnitude as to lay upon the logical and semantic resources of the English language a heavier burden than they can reasonably be expected to bear.
Or to put it another way, they are simply wrong.
Because while genre definitions are fuzzy enough that there is no complete agreement, the level of wiggle room is finite and some books just do or don't fall within a genre definition. Oh, you can have all sorts of fun arguing about the margins, but the definitive texts and definitely not texts are clear. As long as those admittedly unusual (I feel sure genre fans know they're not screamingly rare though) occasions exist, it seems erroneous to me to declare the author/publisher the clear single authority.
And speaking of arguing about the margins...
Many of us understand SF and Fantasy to be mutually exclusive sides of the same speculative coin. Not a spectrum. Either the speculation takes places in our reasoned universe or it does not.
So when someone calls something "science fantasy" this may sound like brunch, it is really like saying dinner-dessert. "Fantasy" means we have gone to a place where the impossible happens, and that essentially demeans an SF title as being so poorly conceived that it is just crystal balls and witches.
It is fundamentally impossible for me to find a definition of reasoned universe that admits ghosts, telekinesis, telepathy, extrasensory perception, energy fields created by all life that bind everything in the universe together, and shooting lightning out of one's fingers in a galaxy far, far away, but which does not admit large swathes of fantasy.
Also as someone who has at times just had a giant dessert for dinner, the idea of dinner-dessert is not as incoherently illogical as you find it.