You're assuming that these groups have a centralized command structure. Sure, some of these guy could be characterized as "militia," but I'm betting a good percentage of the guys actually shooting at US soldiers are local hotheads who see this as a chance to stick it to the occupiers. What we're seeing, though, is that a lot of the fair-weather Jihadees are discovering that shooting at Marines is a good way to end up in a shallow grave. It quickly stopped being fun for them and a lot of these guys are simply melting away, back into the civilian population.
I think it's a very real possiblity that the guys who took this truck driver hostage now find themselves in well over their heads. Holding onto the hostage gets more and more dangerous every day, as American troops continue to close in. Killing him would make them all marked men. So, they're trying to save their own skins. Hopefully, they're smart enough to realize that keeping this guy alive is the only way for them to get out of this situation alive.