this is because xscreensaver-extras is not in the default install. but it looks just like a bug. IMO, we should do the same we do for backgrounds: include a handful of screensavers in the xscreensaver package, and move the rest to extras.
So I talked to Bryan about this and he doesn't think it's that big of an issue. We can always address it again later, if we determine that it really is a problem.