Actually, your subscription only pays for your access to the game itself, it does not entitle you to post as much as you like on a forum that Square Enix owns (and it especially does not entitle you to come here and start ranting and raving about it). This forum is not a fansite, it's owned by SE, they can implement and enforce whatever restrictions they feel like.

The posting restrictions were only added because of the ridiculous spamming going on in the old Beta forum when the game was still in it's testing phase - so really, blame other players for this, not SE. Just level up in game a bit more and the forum will become more accessible to you (the forum restrictions are stated in this official thread here.)