The ultimate answer :
1. Porn is not news.
2. Porn is not educational; (sex education is educational and therefore not porn
)
3. Porn only appeals to one's prurient interest.
That's all they need to beat any legal challenge for removal of porn; all other content
falls more neatly into free speech rights.
4. Advertisers don't want to be associated with porn because of 1,2,3 listed above.
End of story.
.