Exactly, bro. Quality is subjective, so it's up to the owner of the website to determine the quality of their own content based on their own standards and goals.
As a user doing the search, I can't trust the opinions of the website owners, but I have to trust someone. I want to trust the search provider, particularly because I can easily switch if I choose.
and website owners better do their part too, by tagging their content correctly and putting out only the highest quality material.