It was added because the Beta test forum degenerated into a mass of stupid 'me too' spam threads with the mass influx of testers in phase 3, so in order to try and bring order to the chaos they added posting restrictions to the Beta forum. Presumably wanting to avoid a repeat of this nonsense, they added these restrictions to the official forum as well when ARR launched and the Beta forum was closed. Some of the restrictions are a little tight, but they're not too difficult to live with.