Was checking out this video and was surprised at the small caliber gun protecting the tail is the weight factor of a .50 cal that critical they can't use one.
http://www.liveleak.com/view?i=20d_1205419520
Could be mission specific. As I understand you are not allowed to shoot AT a person with a .50 cal machine gun, or there are limitations, just as there are limitations regarding what ammo you can shoot. If they are not expected to engage vehicles, might be something the JAG required.
Additionally, a .308 is not such a 'small caliber'. I certainly wouldn't want to be on the business end of that.