That is 'working as intended' I'm afraid - players only have a limited amount of posts you can do on the forum each day, with the amount based on your in-game level. The official forum posting restrictions are listed here.

As to why the forum has these restrictions, it is due to back during the later stages of the Beta Test when there was a mass influx of new players joining who, in their excitement and eagerness to start testing, flooded the (now shutdown) Beta Test forum with spam and flaming messages, which caused SE to implement similar posting restrictions there. Because of this, when ARR launched it's highly likely SE implemented similar restrictions to the official forum as well to ensure that nonsense was not repeated.

Yes it can be an inconvenience but really, it's easy enough to deal with. Your particular issue due to not having posted for a while probably has similar causes - to stop people flaming the board with stupid "I'm back!" threads.

Basically, it's SE's forum, they own it, they can make whatever rules they like.