In general, cops directly prevent almost no crimes. Their job is basically to respond to crimes and either stop it in progress or find and punish the perpetrators. If they have a prevention effect it is from deterrence.

How many beat cops do you think have been patrolling at night and come across some shady character casing the neighborhood? I'd bet every single one except for maybe the cops in Malibu.

Regardless, there are cops whose jobs are to identify people who may be planning a crime. So my question stands - any concerns with surveilling those people?