they dont support the current nano with it already but an old one?!
I'm going to guess that since the nano has a flat bottom with the clip on the back, it's perfectly fine standing on it's own... all the other iPods are way taller than wider so they need a snug fit to ensure the connector doesn't get bent as the iPod leans.
The new nano is just as tall as wide and has some depth, doesn't need anything.