I don't think they are banning 'porn'. My guess is they are trying to protect users from malware.

That said, it is crystal clear that Skype is probably the most heavily monitored form of IM/video/audio communication along with (plain text) email and Apple's Facetime/iMessage[1].

[1] http://www.zdnet.com/apples-imessage-encryption-claims-refut...