Like it or not, mercenaries and profiteering have existed for as long as war itself. A lot of guys get out of the military and find that they miss the lifestyle, the tempo, or whatever. Or they simply find that they need a job and their prior training makes them far better suited for work as a military contractor than as anything else. PMCs offer them the opportunity to use their skills for in many cases numerous times more pay than the military provides.
I wouldn't worry about American military vets going over to an enemy nation and fighting against the US. Getting a well-paying job with a PMC and fighting against the US are two things with eons of daylight between them.
Mercenaries and profiteering have existed for as long as war itself. No argument there. That doesn't make it right. We should strive for less war, not wars fought by people who will fight them for enough dollars.
Another thing is the profit incentive firms like Blackwater have to keep the war going. More war, more profit.