Haditha is not My Lai

Duh. One of the many reasons, so well spelled out in the article you'll see if you click the link below, is that the enemy is not obeying the laws of war but dresses like civilians. That all but guarantees that civilians will be mistaken for combatants.

"[T]hose who wish to sit in judgment over the Marines at Haditha (before the investigation is even completed, mind you) manage to put on the blinders and refuse to consider how the actions of the terrorists almost guarantee these events will happen."

Haditha, the Laws of War, and the Terrorists

Another reason, as the author alludes, is that the investigation of My Lai is completed. The investigation of Haditha has just begun.