Here is an interesting read. Kathrine J. Chapman, The Untouchables: Private Military Contractors’ Criminal Accountability under the UCMJ, 63 VANDERBILT L. REV. 1047 (2010).
The author argues that:
PMCs must be held accountable for their criminal actions, not merely to provide personal justice for those injured by their crimes, but also for the strategic objectives of organizing the U.S. military’s
available manpower effectively and retaining the support of citizens both domestic and abroad. At the same time, it is simply impractical to bring criminal sanctions against all PMCs for every possible crime
that they might commit. Criminal sanctions against contractors should, at the very least, reach egregious crimes and should focus on quasi-military PMCs. Operating at the battlefront, quasi-military PMCs pose the greatest threat to the U.S. military’s ability to control the contingency operation. Additionally, because they bear arms and wear uniforms like members of the U.S. military, the local populace is more likely to attribute their actions to the U.S. military. By providing justice for victims, criminal sanctions will further the strategic goal of winning the locals’ support and trust during counterinsurgency efforts.
The author argues that:
the UCMJ can be applied constitutionally if three limitations exist: (1) its application must be stated expressly in the contractors’ contracts; (2) it can only apply to quasi-military PMCs; and (3) it must, at least initially, be restricted to noncapital crimes with civilian analogues.