It was because back during the Beta Test the Beta Test Forum became inundated with lots of new testers joining who were a little bit too zealous and assailed the Beta forum with spam, "hello world!" and flame posts, and made the Beta forum a nightmare to use, so SE implemented those restrictions there, and then, once the Beta test was completed and the game was launched officially, they must have decided to try and avoid a repeat of that nonsense by implementing similar restrictions here on the official forum (which until that time was for the original version of FFXIV that ARR replaced).

Sadly, once again a minority of foolish people ruin things for everyone else: "this is why we can't have nice things" and all that.