It's always been my assumption in ED that PvP exists, because damage is simply a function of getting hit.
Open PVE would require checks whether fire came from NPCs or CMDRs, and how much. Imagine a hazres or CZ with NPCs and CMDRs shooting you. For each shot, (for Open PVE to work), there would need to be a check where it came from, who shot it, and whether it should count or not.
Instead what we have now is simply calculate damage from impact (and of course, a bit of did you cause that damage? if yes, here's your bond/bounty, but that's primarily "did you cause damage in the last 15 secs?"), and modes are nothing more than "connect with everybody", "connect with friends" and "connect to no one"
Open PVE would be fundamentally different from the existing ones, and the question is even whether such checks are feasible and still keep the game performing as we would expect.