AgileApps sandbox namespace issue

The wiki states that the java class namespace must be unique within an AgileApps platfom.
A sandbox is created within a tenant (let say ParentTenant), and so a sandbox tenant is created on the same AA platform.
In my mind, a developer should be able to create a new java class on the same namespace as the ParentTenant, but AA won’t allow that.
It seems to be a bug.
Can someone shed light on this?