Continuity mistake: In Kandor, Superman punches a hole in the ground and picks up a large chunk of the street out of the hole, but does not move from that spot. The shot changes to the robot's perspective of him about to throw the rock and shows his feet. The ground he just punched and picked up is in perfect condition at his feet. No hole, no cracks, no sign of the damage he just made. (00:41:15)
Chosen answer: Originally, I thought this would have been to keep murder of police off their charges if things went wrong. But then I remembered several cops were shot and killed during the roof top shoot out before the thugs took off in the chopper. What they were most likely doing is waiting till the chopper was a safe distance from the explosives before detonating it, so they would not blow themselves up. Just so happens the cops got away during that time.
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