That feature is system driven and requires dragging to be performed via Windows API. At the moment RibbonWindow doesn't drag that way when "glass" isn't active so that feature doesn't kick in. If "glass" is active it will work.
In version 2012.2 we started work on a new WindowChrome class that converts any WPF Window to have a Metro-style UI. In that, we used some updated designs where it's fully chromed but Aero snap still works as expected. You can see this in the new demo in the Themes section of the Sample Browser.
At the moment, that WindowChrome class won't work with RibbonWindow though. We do have plans to try and move a lot of the RibbonWindow code out and enhance WindowChrome to support the glass features so that WindowChrome can be used to render RibbonWindow and have the features work consistently there.