I believe this has to do with Canon's little quirk of allowing their lenses to be focused past infinity. If I'm not mistaken, I understand this is to compensate for changes in focus settings at different ambient temperatures. So if you simply swing the focus all the way to the end, you will end up with an out-of-focus shot. The best way to focus on stars is to use live view, and do it manually. Especially since you are typically shooting wide open, and there is no margin for error.

Hope that helps!