Security guards, guard the Police!

[Don Corleone]

I suspect the reasons for private security guards mirror those of the Pentagon going with 'defense contractors' instead of actual military for conducting interrogations. The police have to follow rules and give potential attackers all sorts of warnings and coddle them, respecting their human rights.

Private security are under no such restrictions. You try to screw with them, they only have to explain themselves to their contractor, who's sole interst is task-oriented... no 'respecting the rights of the criminal' crap for them.

Edit: Removed inaccurate representation of British police policy from another thread.