“Police are by constitutional construct, a reactive force to crimes committed, not a preventive force.”
Where the Hell did you get that from? Enforcement of anything is not after the fact.
Sure, cops can enforce traffic laws which we all must take tests to prove we have a working knowledge of them before we can drive. Of course cops should arrest someone for an obvious crime which someone has committed while being observed to do so, but enforcing laws is a reach. It may be a phrase which we've woven into regular speech patterns today, but I do believe the more accurate description of their charter is to "uphold the law". There is a huge difference in the arrangement of the words.