I think that in most jurisdictions seriously planing a crime is a crime in itself.

>A bit like the local nightclub near me has a police car chilling in front of it every Friday night because there are always drunks slugging it out.

And what happens when said policemen block the entrance of the club and start patting down everyone leaving for pico or weed?

A police presence can calm down an area, but also can be used to harass the population there.

Rules of engagement matter.