A bank robber caught in act would be arrested. The USA will decide whether to proceed by filing a criminal complaint or take it to a grand jury for indictment. In either case, the decision to go forth rests with the prosecutor. The key point is the FBI does not take matters to the grand jury nor does it issue indictments.
You’re saying the FBI cannot then legally arrest Clinton, if they wanted to, for Federal Crimes like that has been reasonably shown in her email handling?
I realize the politics of this and the FBI DOES and CAN arrest, detain and hold for indictment by the DoJ. It is up to the DoJ on what to do from there, but don’t tell me the FBI does not have any power in this.