Makes perfect sense to me, it means that people who are just signing up to the forum to argue, and arent prepared to support the site financially dont get access to it.
Having it behind closed doors also keeps the likes of google out of course, and I doubt IB want arguments to be presented to the search engines.
If arguments interest you then you can pay a nominal fee to read them, if they dont interest you enough to part with a bit of money then surely they dont interest you enough to let it bother you so much you post about it?