The ballistic helmet doesn't protect against shock damage - just ballistic and bleed. The motorcycle helmet does protect against shock and thus should prevent you from getting knocked out easily.

Source: http://www.reddit.com/r/dayz/comments/1tp2za/how_to_mug_people_as_a_fresh_spawn/cea6vgx

Seems to be the same issue as http://feedback.dayzgame.com/view.php?id=547

Duplicate of http://feedback.dayzgame.com/view.php?id=547

Can confirm the original issue for the full inventory - I do not have access to a case to reproduce that.