It is very much the prosecutor's call to make. Prosecutors aren't (at least in theory) supposed to charge the accused with as many crimes as possible. It is the job of the prosecutor to ensure that justice is served, whatever that might mean given the facts before them. Sometimes that might mean throwing the book at the accused, but in other cases it might mean dropping the case even when it is clear that the accused really did break the law. Unfortunately prosecutors are rewarded for successful convictions, so it is not surprising that prosecutors act as though their job is to charge the accused with as many crimes as possible.