As far as I can tell the immunity claim is because they are involved in a theatre of war.

Who grants it? The US government.

The question is why do they grant it.

I'd like to see Blackwater's case tested in court. It's a facet of democracy, testing things in court, especially large companies. Granting immunity to whoever is a facet of the type of regime getting rid of which is supposedly the point of the war.

Let's turn the tables for a minute. Suppose the Iraq government granted immunity to companies to kill Americans?