The FBI isn't a large part of the reason the US has so many people in jail & prison. Mandatory minimum sentencing laws are about 3/4 of the cause, along with treating drug addiction as a crime instead of as strictly a health concern is most of the rest of that (with all of it broadly falling under the war on drugs). The get-tough-on-crime (by putting everyone in prison) era, during the Reagan / Bush / Clinton years, is the root.