I think you are still missing what I meant.
I think it's a Content vs Pornography distinction. If we wanted to test it, I think the question to ask would be are they blocking explicit text as well, say erotic stories, or even just swear words? I know that some portions of their platform do automatically censor certain words (for example website comments), so it would be interesting to see to what level that policy is policed.
Oh---I did miss your point then, but I think the text/photo distinction would actually be somewhat easier for them to defend than a content/porn distinction; as I understand it, porn is content, as a matter of very well-settled caselaw.