Well, if it were a security issue as you suggest, wouldn't others be equally affected by it? Instead, as I mentioned, everybody else can post iframes just correctly. My attempts to do so result in an empty space of specified size. In fact, it doesn't work even when I do as little as quote somebody else's iframe.
Edit: Well fuck me it works.
It still doesn't seem to show up correctly on previews though, and I swear it didn't work in a post last time I tried it.