The mission may be optional... And the PR is only going to help sales, as if it needs it. As someone mentioned (I think in the video comments?), how is it any different from mowing down a load of civvies in GTA?
Their official response:
"The leaked footage was taken from a copy of game that was obtained illegally and is not representative of the overall gameplay experience in Modern Warfare 2," an Activision spokesperson told IGN.
"Infinity Ward's Modern Warfare 2 features a deep and gripping storyline in which players face off against a terrorist threat dedicated to bringing the world to the brink of collapse," the spokesperson added. "The game includes a plot involving a mission carried out by a Russian villain who wants to trigger a global war. In order to defeat him, the player infiltrates his inner circle. The scene is designed to evoke the atrocities of terrorism."
Right now, Activision is saying players may choose "not to engage in the gameplay." That could mean choosing not to fire your weapon or the possibility of an "opt-out" from the level entirely. We won't know until we play for ourselves.
"At the beginning of the game, players encounter a mandatory "checkpoint" in which they are warned that an upcoming segment may contain disturbing elements and they can choose not to engage in the gameplay that involves this scene," Activision says. "Consistent with its content, the game has been given an "M" for Mature by the Entertainment Software Ratings Board. The rating is prominently displayed on the front and back of the packaging, as well as in all advertising."