I saw that too.  It's up to the user if they want to clone.  This would be no different than, say, the `center` property of the [EllipsoidPrimitive](https://github.com/AnalyticalGraphicsInc/cesium/blob/master/Source/Scene/EllipsoidPrimitive.js).

The function had to clone because the primitive could be victim to side-effects, but with a property, this is a more explicit.