Thus far, no criminal charges have been brought against her rapists. There are laws in place that limit the ability of the Iraqi government to take action against contractors in Iraq. I do believe criminal charges can be made if a crime is committed by or against a US national, but I don't know why this has yet to be pursued. It might have something to do with her KBR suddenly "misplacing" key contents of her rape kit. What would you expect from a company whose management locks a rape victim in a storage bin to cover up her rape?
Now, if your question is the greater one of WHY this happens—why do contractors feel the need to include this clause in their employment contracts to begin with, and why is the Department of Defense itself opposing this bill because it is too difficult to enforce—I have no idea. Apparently national security hinges on the inability of rape victims to bring their rapists to justice!