Because had a soldier shot the same person, he would have been in the brig, and it would be very publicized, he'd be court marshaled and put in jail.

NPR: Oversight of Blackwater a 'Grey Zone,' Author Says

Monica's presentation was enough to persuade the prosecutors to offer her an extraordinary deal: complete transactional immunity from prosecution for both Monica and her mother, which means that nothing Monica or anyone else says can be used to put her in jail unless she fails to testify truthfully when she finally has her day before the grand jurors.

CNN: Tick, Tock, Tick... ...Talk