To see why, we must consider what posters are hoping to accomplish. If they want to communiate, discuss, argue, criticise, make sense, provide food for thought, ask questions, get help, make friends, have a nice time talking, or anything remotely normal like that, then they will need to communicate to accomplish their ends. In other words, failing to communicate effectively is inimical to their own intentions. And, as I've said before, life strategies that fail *by their own standards* are morally wrong.
Note especially that people don't do things for no reason, so saying posters haven't any goal won't suffice. And writing gibberish to annoy people, obviously won't do to defend the morality of ineffective communication.