Violence is expensive. If a police agency offered this service, their rates would have to go up. They would have to pay higher rates to their officers for the risks they would take in taking others' property by force, for example. The higher prices, along with the fact that most people would not want to support such a service, assuming most people are opposed to stealing, would result in them having a continually decreasing market share. The honest police agencies would have more customers, cheaper rates, and would be powerful enough to withstand any aggression from rogue police agencies.
Plus, if a police agency violates your rights, you can go to courts. If the courts are free market as well, they need to be known for rendering fair decisions to attract customers.
“With laws shall our land be built up, but with lawlessness laid waste.”
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