Why does the way private military companies (PMC) work cause them to be considered controversial? They come from law enforcement and show that they can apply the same skills to military operations.

They come from the largest security firm, G4S, which focuses on prisoners and detention.

They contract their services as security guards but they perform military duties.

They are set up much like the Pinkerton agents during the Civil War where they use intelligence and secrecy to provide security.