You're often killed by them while you're engaged with an enemy in the foreground, and most of the time the Sniper is so far away it feels like you couldn't have dealt with him even if you didn't have enemies nearby. In fact, the Sniper's goal is to create that relationship: he specifically wants to fight enemies outside their engagement range, because that's his primary advantage.

Which brings us back to the Sniper Rifle unlockable. In an effort to reduce some of the aggravation that other players feel towards the Sniper, we chose the goal of designing an unlockable that encouraged the Sniper to get a little closer to his target. We want him to give up some of his primary advantage in return for something else, so that enemies he kills feel like they were engaged with them, and feel like they could have survived if they'd just managed to fight a little better.