Question about Military Contractors in Iraq

Are they exclusively held to their current contract with the US Governemnt(our tax dollars)? Or as a private corporation, are they free to enter into contracts and/or seek employment elsewhere(ie: the highest bidder)?
Looking back at the history of mercenaries employed by a national interest, it wasn't uncommon for some to actually switch loyalties during the middle of a conflict to chase bigger bucks.

Does anyone know if something like this is possible with the contractors we employ ?

Second, they can just make working for non legitimate organizations (terror groups, enemy nations) illegal. That's why mercs often get hauled to court by their home nations. Sovereignty trumps the dollar bill.

Like, Gurkhas. Or, they want auxilliary allied forces. Like, Canadians and Australians. Or, they want to recruit foreigners from poorer nations who will serve under your flag and be held accountable to your military discipline. Like, the French Foreign Legion, or the U.S. Army.

Frankly, Iraq is only an extremely recent revival of "mercenaries". The thing is, they're NOT mercenaries in the classic sense at all. They're security companies. These are highly paid security guards with military gear and experience. They are not freelancers; they are rented on longer term contracts. Now Iraq could try to phase them out somehow with its new sovereign government but um, good luck. And at least the security companies are on the right side, however many civvies they shoot up (everyone does that anyway).

Edit: Many commentators know all this and call them mercenaries anyway because it's a more insulting term.