First, there's a distinction between unelected federal prosecutors and your local, elected prosecutor (your district attorney). Federal prosecutors don't have the latitude or political independence to change charging policies (e.g., setting a blanket policy to decline prosecution on simple drug possession charges). Your local district attorney can do that, but note that this latitude doesn't extend to the front-line prosecutors in every local DA's office. Only the elected person -- the DA -- gets to set that policy. If you get hired as a front-line prosecutor and the DA who runs your office wants to hammer people on marijuana charges, you either get with the program or you'll lose your job. But, if you're in an office where the elected DA is trying to put decarceration policies into practice, then yes, you can choose to decline prosecution on practically any case.
Here's a policy memo from a DA running this sort of program. Skip down to page 46 of the PDF if you want to see highlights of other DAs doing similar things, and skip down to page 55 if you want a look at what types of cases will lead to the DA's office declining prosecution (or using a diversion program as a first step rather than incarceration).
You can be a prosecutor who doesn't prosecute people, if you're an elected one, right?
You're on the right track.
First, there's a distinction between unelected federal prosecutors and your local, elected prosecutor (your district attorney). Federal prosecutors don't have the latitude or political independence to change charging policies (e.g., setting a blanket policy to decline prosecution on simple drug possession charges). Your local district attorney can do that, but note that this latitude doesn't extend to the front-line prosecutors in every local DA's office. Only the elected person -- the DA -- gets to set that policy. If you get hired as a front-line prosecutor and the DA who runs your office wants to hammer people on marijuana charges, you either get with the program or you'll lose your job. But, if you're in an office where the elected DA is trying to put decarceration policies into practice, then yes, you can choose to decline prosecution on practically any case.
Here's a policy memo from a DA running this sort of program. Skip down to page 46 of the PDF if you want to see highlights of other DAs doing similar things, and skip down to page 55 if you want a look at what types of cases will lead to the DA's office declining prosecution (or using a diversion program as a first step rather than incarceration).