I think the "cost" argument here misses the point slightly. Law and order has value, in and of itself. There's a reason why the punishment for stealing $100 is not a $100 fine. We all benefit from a society where no one steals things.

Not to strawman, but for instance, I see no ethical argument that homicide is less serious if you murder a person with a net negative cost to society. Neither murder nor theft are wrong simply because they're expensive. They're wrong without the context of cost.

In both cases, cost effectiveness isn't what I would consider the most important thing. However, I 100% agree with you that Walmart is certainly benefitting from the public police force enforcing private security. It's a sticky issue without a clear solution.

Crimes are crimes, but there's also a reason we install locks and alarm systems in our homes. By maintaining secure systems, we reduce our vulnerability to crime and our cost to everyone else. I feel like Walmart should do the same.