This only applies to documents released for review.
You cannot completely delete those documents because they are generally being used by several users, and, this way, we avoid deleting documents and their history.
This is also why you, even in the role of approver and as admin, cannot delete the document.
Each user can move the document to trash but only to his/her own trash.
All other users, however, continue to see the document in their document lists and can work on it as usual and collaborate with each other.