Surely it's not so they can look "less military" when patrolling civilian streets. It better not be.
This is a terrific site on the topic:
Iraqi Ambush: bad Tactics, Techniques, Procedures and Equipment
One of the killed and one critically wounded guy were in my old Army Guard unit. They were friends of mine. I'm just numb, I've been talking about humvees being useless for combat duty for years and now this.