Thanks for the link to the discussion on MDL-27191, and that clears up the mystery for me! I can see the dilemma if you import/restore something and suddenly your grade book is destroyed, but that is exactly what happens to those of us hoping to bring in the gradebook.
In that discussion David Mudrak suggested, “Shall we consider a workaround solution (gradebook categories export/import independent on backup/restore?).” Yes, yes, yes! Andrew Davis mentioned a use case where his school has departmental guidelines for grade categories that are independent of content, and I think this is the case nearly everywhere, especially in universities.
In the meantime, could there at least be a notice? “Grade categories will be lost/preserved.” Or, “If you would like to import/restore grade categories, your destination course must have no gradebook and your import/restore must be a full restore.”
This might help avoid the numerous issues that have been discussed here and in the bug tracker.