Well, it's not like overzealous prosecution is unique to the feds. The DA in the Duke lacrosse thing was a local, for example.
I'd imagine that with broadly written laws that carry stiff penalties is that you either push for uniform enforcement where everybody gets the book thrown at them, or leave it to the discretion of the DA's, and wind up with selective overprosecution. I'm not really sure what the incentives are like for prosecutors and whether they have any reason to worry about overcharging someone if the case doesn't draw a ton of negative publicity.