the Geneva Convention -- rules which prohibit torture, destroying "protected objects" (such as churches or mosques), or, say, shooting a bunch of people who are waiting to catch a plane. The games observed, which include 24: The Game, Call of Duty 4 and 5 (we assume they meant Modern Warfare 2) and Metal Gear Solid, were largely panned for allowing the player to simply ignore the Geneva Convention's statutes.
According to a BBC report on the study, the two groups explained that while players of these titles may never become soldiers, these war crime violations could "influence what people believe war is like and how soldiers conduct themselves in the real world."
In short, you should feel bad about your recent airport murder spree in Modern Warfare 2. Not for the moral implications of killing dozens and dozens of innocent people, mind you -- but because it was against the rules.