Supposedly they are to leave you alone, unless you're actually carrying powerplay material. But maybe some of the code that originally made them shoot any hostile ship (which is any ship from another power) survived.
I think this may be different in Strongholds. I definitely remember being shot at by Power NPCs before this CG, while not carrying Powerplay cargo. I think the popularity of both this CG and PP2 have made it more likely for CMDRs to get shot at.