The Marine definition of "self-defense" is a little broad. Someone shoots at them, they level entire blocks. Remember, under posse commitatus, they can't patrol to restore law and order, but they can defend themselves if attacked.
That, and they're amphibious troops by definition and equipment, so they're perfect for SAR in Lake Nagin.
Methinks some gangsta's are gonna have an epiphany soon. I would not expect to see standard close air support. These will be up close and personal. No matter who they give their soul to, their @** will belong to a Marine.