You have to look at it from the perspective of the guy who's up against 4 players... The odds are already stacked against the person and they manage to sneak up on someone, even though they can be seen through walls or have the 4 man team co-operate by grouping together to make it near impossible for the antagonist to get a kill... In a situation like that, If the person manages to get that close to someone WITHOUT BEING NOTICED, they they deserve the stealth kill, IMO. Without the threat of the instant kill(THAT'S PRETTY EASY TO AVOID CAUSE OF ALL THE FEATURES), things would be too unfair for antagonist players. If devs nerf the dmg, and they manage to land a grab on someone who's close to a team mate, that's an automatic death for the antagonist.
Things need to be balanced for everyone, not just the co-op team. For every situation, there should be ways to counter them.... that's why game lets you counter grabs and has a feature that make it difficult for anyone to sneak up on you (seeing enemies through walls). If anyone gets stealth killed, it's that player's fault and they/we should be rightfully punished for it. *Map awareness* is an important skill in any online game. A competent player should always be aware of their surrounding, learn how to play *cooperatively* with your team, and use the feature that make it difficult for anyone to sneak up on you.
As for the ammo; you should always keep count of how many you have. Even if you're a sniper, your team mates aren't the only ones doing their job, you should be contributing by doing your part too, all the while *paying attention to how much ammo you have and how you're killing the enemy so you don't run out.* Just like getting snuck up on, if you run out of ammo, it's the players fault for mindlessly shooting without keeping track of their resources. I play sniper too, It isn't that different from any other class.
EDIT: And everyone has the same available options for each side. I've snuck up on antgonist players, too. Stealth killing isn't something that's exclusive to them.