It would appear to me that as long as the Private Contractors in Iraq and Afghanistan are perfoming defensive tasks protecting persons, assets, the like, and are subject to US Military law as they are now.....they in no way fit the legal definitions of a "Mercenary". Thus, to call them that, is incorrect.
If the boys were laying on a roof top and offing insurgents who posed a threat to them or what/who they were assigned to protect then they were well within the guide lines.
You may not like that situation but that has nothing to do with the law.
A true Mercenary owes his loyalty to his pay check and is for sale to the highest bidder regardless of idealolgy.
You want to suggest to some of the Blackwater guys they could make more money working for the insurgents? I would pay a lot of money to see that ass whupping!