I don't regard the MA as one big one-on-one duel, with every fight calculated to measure and highlight any and all differences in skill between pilots. When I'm flying a mission in the MA, I try to shoot down enemy planes while not getting shot down. Sometimes I'm supporting a larger effort like an attack on a base, or defending a base, or whatever. Sometimes I'm just dogfighting and having fun.
I personally have no compunction against shooting down a landing plane. Why would I? He's the enemy. It's not about whether it's a "fair fight" or not. If that's what you insist on, go hang out in the DA and go 1 on 1 against people.
If you're going to land, land at a base that isn't under attack. That's part of thinking and planning and acting intelligently. It's part of having good SA. If the other guy didn't plan where he was going to land so that he would land somewhere safe, and if he just went ahead landing even though his base was blinking and I was on his 6, well too bad, he's going to die, and that's all there is to it.
I really don't have too many unwritten rules in the MA, other than that I think it's exceedingly lame to shoot at a bomber or a fighter someone else has already lit on fire, or which is falling from the sky with no tail or wing, or whatever. Blatantly shooting at an airplane which is already guaranteed dead, just to try to get the credit for the kill, is about as lame as it gets.
But killing a guy who tried to land with an enemy right there? That's fair game, and the landing pilot is stupid, not the killing pilot is dishonorable.
My opinion, and I'm sticking with it.