Bitcointalk to other Cryptocurrency forums is same as "Bitcoin" to other Altcoins "Father Figure" so Bitcointalk doesn't need to ban posting or talking about other forums. and it is not necessary either. Bitcointalk have it's place booked among bitcoin and crypto community. it cannot be replaced or overtake by any other forums.
Besides Bitcointalk is a free place with freedom of speech. everyone can talk and post about anything, promote their projects (if bitcoin and crypto related) freely without any restrictions. Bitcointalk don't even moderate scams. then why would it ban another forum?
I am a member of other popular forums and they do not allow mention of competing forums
that's a pretty dick move. but they have the right to make their own rules. who cares.