This is serious business.
I have serious news. (thread derail gogo tacgnol!)
As a Bst, I always try to respect people's NM claims, yeah, even if the thing goes yellow. Basically, is it that much to ask to let someone finish the fight? If they die, then all bets are off, go ahead and engage the mob. Similar concept to Sky (or how it should be ideally.) If an alliance wipes, that fight should be considered over.
If you want to be a jerk and steal NM's at like 10%, go ahead and try. Just remember karma is a bitch, and often does come back to bite you in the ass :p
Actually POL just updated their little manners blurb, and pretty much says what I just summed up there. Be respectful to other players, and most the time, you'll get similar treatment back...unless your just dealing with a complete tool.
Sorry you lost your mob.