Originally when version 1.0 was active, the forum was not restricted, but during ARR's Beta test, the Beta forum became inundated with spam and troll threads, so to try and bring order to the madness SE implemented similar restrictions on the (now closed) Beta forum. I presume that must have then spooked them and thus they decided to head off any possible trouble with ARR's launch and implemented these restrictions on the official forum accordingly.

It's always possible they may remove these restrictions at a later date as well.