I'm seeing a lot of this crap on Uncharted 2 lately...
I have two builds on Uncharted; Down the Irons & Bandoilier, or Down the Irons & Deposit.
A quicky for people who don't play the game; Down the Irons means when you are firing from the shoulder (ie: aiming) with a long gun like the AK47, you are more accurate. The AK47 on Uncharted is so pathetically inaccurate that without Down the Irons it has sod-all chance of a medium range kill, let alone long-range sniping. Bandoilier means your clips carry more bullets (not a huge amount - 5 to 10%) and Deposit means you get more points for objective based medals like capturing the hill on King of The Hill matches.
The people who scream abuse at me usually run Situational Awareness, combined with either Rocket Man, Launch Man or Revenge.
What do they do? Situational Awareness makes the enemies appear on your screen (this game has no radar) so you can track them. This only happens when stationary, but it's not hard to stop-start.
Rocket Man gives you more Rocket Launcher ammo (a one-hit kill weapon). Launch Man gives you +50% ammo with the Grenade Launcher (a 2-hit kill weapon) and Revenge automatically drops a grenade when you die.
In short, I take a loadout you can get at level 14, go against people with a level 46 loadout (I am level 43 by the way) and get endless abuse from the cheap little twats. Usual accusations include 'cheating' for using 'revenge' grenades on them without the Boost (this is done by throwing a grenade at them or the floor before they can kill you. It isn't cheating). Others include Stealth-killing (yes, I have snuck up on people who can see where I am...) or performing Pull-Downs (an instant kill done by hanging off a ledge and waiting for an enemy to walk up to the ledge, then meleeing them).
So, camping with a Sniper Rifle behind cover so you can't be seen, and using a Perk so you know where every enemy on the field is, then sniping when they come into view is not cheating... but killing that Sniper in a way that can only be done if the Sniper is stupid enough to leave himself exposed and vulnerable to the most BASIC of attacks is a form of cheating.
Some people are just retarded.