For the evidence to be illegally obtained, a law enforcement officer has to violate the 4th amendment in obtaining it. In the case of an armed robbery, I can ask the clerk for a copy of the tape, which they'll give me, which is legal. If they don't/won't, I'll obtain a warrant, which will allow me to have a copy of the tape, either way, perfectly legal.
As for a private company recording their premises, I think they're covered because it's private property