Route is by definition administrator-level functionality -- the whole purpose of the Route command is to allow the workflow transitions to be bypassed and any workflow state to be set. Consequently, the permission system does not go to a granular, state-by-state level.
If you find that there is a need for content editors and authors to be able to transition from one or more states back to draft, then just add workflow transitions to accomplish this.
However, consider what the actual use case is for allowing this. Is this required to "fix" an asset that has progress too far in the workflow, perhaps by accident? This is a use case for a content administrator to use Route. Is this required to "unpublish" content that was perhaps seasonal? The Basic Workflow has a "Retired" state for exactly this reason. Basic Workflow also allows content to be transitioned from Archived back to Draft.
