The thing is though, you wouldn't turn up to airsoft with a watergun, if you don't have the right equipment then you can't expect to be as successful as someone who does.
I don't think it's unsporting, if he had an MED he could've just moved to a more suitable position to shoot from. It's just like the bang rule, it's a situation that will cause arguments, best way of avoiding it to have the right equipment and shoot the player in the back of the vest or somewhere that won't hurt.
If you ran into a room in cqb with a sniper rifle shouting take the hit to anyone inside, you'd get shot and look like a knob.