Cops and prosecutors are coworkers whose jobs fundamentally depend on cooperation from the other. They aren't and won't ever be adversaries. A given prosecutor

Cops and prosecutors are coworkers whose jobs fundamentally depend on cooperation from the other. They aren't and won't ever be adversaries. A given prosecutor having a personal working relationship with a given cop certainly makes this worse but it isn't the core constraint. An out of town prosecutor will still consider police generally to be allied peers. They still have to go home afterwards and have a working relationship with the local police, who certainly consider the cops a few towns over to be part of the same family and will consider prosecution of one of them an affront to all.