I can not find a reference either but I have seen the same poster. IMO I think it's bullshit, I've seen stories of shop keepers killing fleeing robbers and the shop owner is almost always charged. The reason I LIKE that idea, is that if someone performs an armed robbery, it's probable they may use excessive force on yet another innocent civilian. Eliminating the wrong-doer would eliminate the chance of this happening, possibly saving ones life. If someone is going as far as armed robbery, police and government law enforces usually consider them armed and "dangerous" for that reason, but they aren't dangerous if they're dead.
It also allows said shop keeper to kill the thief before their own life is possibly ended, they have that right to defend themselves. I don't see it as cold blooded murder, as they are potentially saving other civilians lives.