Even if I were to assume that all police officers are perfectly honest and reasonable, virtually anybody who is choosing to break the law would also choose to not voluntarily subject themselves to the consequences. Nobody chooses to speed down the highway then calls the local cops to confess and accept the fine. And if police sirens were nothing more than polite requests to pull over, not demands backed by implicit threats of violence, virtually nobody who is choosing to break the law would heed those requests voluntarily.