by Michael Hughes.
This is a known issue (see https://tracker.moodle.org/browse/MDL-42585) as Moodle currently doesn't make a distinction between being able to view (or edit) grading information (from an administrative, not the lecturer sense) and grading an activity (mod/assign:grade capability).
It should be fixed in 2.7, but they declined to back port it to 2.6.