Mercenaries needs to make a living also....
But really... It's not the US military the mercs work for... They work for... Well, anyone really. Mainly businesses, mainly private contracting. Someone gets kidnapped, you call in the mercs. Someone wants to blow up your TV factoiry, you call in the mercs....
You can trust them because they work for a living, they're not biased by religion, color (except that of money), creed, age, ethnicity or any crap like that... They're mainly ex-military, and they know how to protect people and places. That's what they've trained for all their lives.
The US military tries to put as much responsibility as possible into the hands of the local Iraqi goverment and security forces. Those forces however, may not always be enough to protect YOUR companys assests, so you go and hire a couple of mercs. They make the difference in case of problems.
I have no problem with that... I even KNOW at least one merc, and I have no problem with him doing shit like that... It's his life so let him make some money playing soldier if he wants to.
And ofcourse the upside is: if the mercs die, you get new ones, and the US military doesn't have a SINGLE casualty if one of these guys gets whacked.... Everybody wins...