Because it's obviously being used in the thread (and newspaper) title for a reason. It may not be illegal here (yet) but it certainly has more connotations than legality for the minds of most people that read it.
that doesn't have much to do with the legality of it - famous person paying for sex with someone above 18 is still news for tabloids even when perfectly legal
you could say it is related to the morality of it... but really it is just something that makes the story/gossip a bit saucier