The answer is: it's an occupation. This isn't war; our men and women are much more exposed to this kind of thing, because we're in someone else's house.
The military lied about this at first; but really, who is surprised by that anymore?
Military Confirms AP's Claim of False Report on 4 Brutal Deaths in Iraq Last Week:
"In perhaps the boldest and most sophisticated attack in four years of warfare, gunmen speaking English, wearing U.S. military uniforms and carrying American weapons abducted the four U.S. soldiers last week at the provincial headquarters in Karbala and then shot them to death."