Straw purchasers should be prosecuted for whatever crime committed by the person they passed the gun on to. I think the reason that most straw purchasers are given a pass or a slap on the wrist is that most of them are either girlfriends, wives or mothers of the criminal.
In the case of Operation Fast and Furious the ATF hired the straw purchasers specifically to buy quantities of guns and smuggle them into Mexico. When gun dealers called the ATF to report those suspicious buyers they were told to make the sales.