The contract with the agency would require the enforcement of the law as a condition of employment and compensation.
Although I have admired his scholarship on other topics, and think I understand where he's coming from on this, I believe Robert Higgs is technically incorrect in saying that "Every cop has agreed, as part of his job, is [sic] to enforce laws; all of them."
My understanding is that police officers in the U.S. have to swear an oath to uphold the Constitution. I'm not aware of them being required to agree to enforce all laws. Certainly the reality is that most officers are selective about when they choose to enforce the law. They don't go after every single person they see doing something illegal.
I believe we should emphasize the existence of this police power of discretion, and encourage them to use it for good by refusing to enforce bad laws. As well as encourage them to join Oath Keepers (http://www.OathKeepers.org).
Love & Liberty, ((( starchild )))