imported post
bossbart,
ANY exception to the preemption law would need to be listed in the law, much as it states that a local unit of government is not prohibited from forbidding the carrying of firearms in the course of their employment with the governmental unit.
For example, if Detroit wanted to be able to enforce stricter firearm laws, there would need to be an exception listed either naming the local government that is not preempted or, as is often done, list any areas exempted by description, eg: This does not prohibit any city with a population over 500,000, etc.
Local units of government still attempt to interpret what ordinances are actually preempted, and they will often do what they want until either someone complains or brings it before a court of law. Remember too that a local PO does enforce state laws too, so if the city interprets state law in a way that is, let's say "novel", then once again, it generally is a court of law or, an attorney for the governmental unit that determines the "correct" interpretation.