There is a big difference between a necro being able to allow his zombies to ATTACK people out of pk vs them being able to be attacked by people out of pk.

I don't think that zombie should join in a pk just because they are grouped. If someone hurts a zombie by mistake (eg if paladin holy words) then fine, let the zombie pile on him, but for the necro to be able to cause his minions to attack by wording does not sound like it is "by design" to me. Maybe "by bad design".