The problem is trying to find an absolute political or ethical principle that will always hold, right now, in an explosively contradictory point of development, history and society. The cops are "bad" always so we shouldn't rely on the cops ever or exploit the ability to pit the police against reactionaries because in doing so we give the appearance and therefore tacit endorsement of supporting the oppressive arm of capital.

It doesn't need to be that complicated, siccing the police on exposed fascists doesn't mean you endorse the police forever and it doesn't somehow seriously contribute to perpetuating the police because you've relied on their existence to confine fascists, they would persist anyway in the absence of large revolutionary changes. Don't rely on them for meaningful protection or assistance or service in any way, but if your expectation is them getting involved will fuck things for people, that's accurate, and if those people are fascists that's a win.