This strikes me as needing explanation. It is like a world for which every choice exists - except the choice to opt out.
There are people who want all the porn - there are people who want no porn - but between a bigger group has some fetishes and are disgusted by others. Why then does the standard negation operator not work on any mainstream porn channel?
I could make some guesses - but you have to admit this is a deliberate choice by the porn search engines - I know everybody on HN knows this is not a technical issue - so I'm asking if this is a known business decision or a known social intervention.