it's a different kind of crime than 3 guys coming in holding up a store - demanding money at gunpoint.
When criminals do that a store clerk or store owner is justified (in most cases) in using deadly force.
When someone comes in and grabs some merchandise off the shelf - the store operators aren't justified in shooting that person. When you multiply that by 15 or 30, you have a "mob" of people emptying the shelves like a swarm of locusts, but the store owner is still not justified in using deadly force. He can't come out from behind the counter and just start blasting away.
The perpatrators know this.
The flash mob muggings are different though... when a mob attacks someone, deadly force is justified by that person to defend their own life, and/or by another person to defend the life of the person being atacked.