That button is for rating the movie/show after you watched it. I want to apriori eliminate items before even attempting to watch them.
True, it does affect their recommendations but still the inherent purpose of a thumbs down is to tell them I watched it but didn't like. I would love for them to provide a "not interested" option as well.
Edit: I continue to have hope that some higher-ups from Netflix are reading these comments, and someday will create a GUI for "pros" and a GUI for everyone else (as it is now), where the "pro" version allows at least some level of customization beyond this dark-pattern insanity.