Imagine there was open only. How exactly would you defend your BGS with PvP?
Given that, as you say, you're part of a PMF and you're been playing the BGS, surely you must already know that shooting player ships, like any other ship that isn't wanted, will only harm your own faction.
There are several cases where this isn't true, though.
1) Trivially: You're not attacking them in your own system (e.g. your opponent takes a bunch of missions for an opposing faction, jumps out of the system, and then you shoot them ; your opponent is collecting bounties in their system to transfer in to your system)
2) The minor negative effect of the crime is still better than the substantial positive effect for the other faction if they got through (i.e. the ship being attacked has a lot of bounties, bonds, cargo, fail-on-destruction missions or data aboard)
3) A War conflict is ongoing and both players are in a CZ
4) An Election conflict is ongoing and therefore combat actions have no effect
5) The other side has multiple objectives and attacking them convinces them to focus on other ones
6) The other side is already piling up NPC murders and crimes versus your faction, so the diminishing-returns effect of another assault bounty is basically zero (and that's
if the target is clean anyway)
7) The other side is full of easily-distracted PvPers who will quite happily spend hours chasing a couple of ships of yours while the rest of your team works unnoticed.
As I said above the opposition is under no obligation to only use "tournament-legal" PvP tactics, so "open only" would be a terrible disappointment for other reasons, and I can't imagine there are many cases where it would change which side won the conflict overall, but to say that PvP is not an effective BGS tool is oversimplification.