That's funny, because this post from Yoshi-P (that is stickied on these very forums, no less) greatly contradicts what you just said.
http://forum.square-enix.com/ffxiv/t...s-Restrictions
Resolving AFK Character Issues
"From several users on some of our more congested Worlds, we have received requests for a feature which automatically logs out characters who are AFK for an extended period of time.
We are actively looking into the implementation of such a feature, but have decided to first concentrate our efforts on increasing the number of Worlds and optimizing the Duty Finder servers."
In short, it is something that they are working on and will be implemented but unfortunately is not on the top of their to-do list at the moment in lieu of these server problems.