I always just assumed that it's because the left arm is a big target if you're facing a player's left side and it's also the arm that has the most exposure from the front since it holds up guns. Would be annoying but still kind of awesome if CS were like Red Orchestra in that you couldn't hold up a firearm for a while if you take a shot to the arm, or IIRC one of the UT mods had you bleeding and leaving tracks to follow. The only thing CS has to discourage reckless tactics is the AWP but it's a shitty solution because it's still cheap enough that people will abuse the hell out of it. Should cost a full $16000