Blame the morons who flamed up the Beta Test forum - originally the main FFXIV forum and the Beta forum did not have these restrictions... until just prior to phase 2 of the Beta test, when there was a mass influx of new testers, who, were a little too excited about being selected and thus spammed the Beta forum with stupid "hello world" threads. In an attempt to bring some order to the chaos SE added posting restrictions.

And possibly in an attempt to stave off any chance of a repeat happening at launch, similar restrictions were also added to the main FFXIV forum, which until then was only available to version 1.0 players. Really, the restrictions aren't too difficult to deal with (really, does anyone need to post a thousand whine threads complaining about how the RNG sucks?).

Besides, this is SE's forum, not a fansite - they can administer it any way they see fit. This isn't a democracy. Basically, paraphrasing Zidane in FFIX: "It's SE's way or the highway."