It's because the massive spamming that went on in the Beta Test forum between phases 2 and 3 of the Beta Test so spooked SE they had to add posting restrictions there first just to bring order to the chaos, and presumably it so spooked them they decided to implement here on the official forum as well upon ARR's launch so as to prevent a potential repeat of that nonsense. And the posting restrictions are technically not invisible, they notified players about it when it was implemented (although the post in question has kind of fallen to the bottom of the pile).

Yes, they can be annoying but the annoyance is mostly momentary, this is SE's forum so they can implement whatever restrictions they like. There are plenty of unofficial fan forums and social networking sites you can use instead like Reddit if these restrictions are too offputting.