Under Score Editor…Staff Settings…Staff…Options…Score Drum Map, the checkbox for “Single Line Drum Staff” is grayed out until a drum map is attached to the track.
This seems incorrect since I don’t use drum maps, and I can work around this by temporarily attaching a generic drum map, checking the box, then removing the drum map.
Interesting observation 
I’ve just checked, and it is the same in Cubase 7.5.30. Not sure if this is, technically, a bug, but if it is, then at least it is a useful one! 
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