This is an age old American tradition of shooting people who try to stow on train cars. There is an image in American culture of the freighthopping hobo who is trying to find a better place to live and work despite not having a dime to his name. Of course in reality many people have been shot for doing that. Property and a few dollars is worth much, much more than a poor person’s life.
They would never win. The police were just doing their jobs after all. So what if a couple of innocent people get shot? After all, just because they are currently innocent, doesn’t mean they aren’t future criminals. So really, by shooting them they make it less likely that they’ll commit future crimes!