>While law enforcement has some potentially excludable components, for the most part it seems non-excludable: stopping crimes and criminals protects everyone.

that is true only in the ideal situation of unlimited law enforcement resources and the type of crimes affecting everybody. In real situation going after specific criminals or crimes [esp. the ones affecting only specific subset of the society] leaves other crimes unpunished and laws unenforced. Thus any real law enforcement inplementation (not theory) demonstrates rivalness and excludability.