Not being able to track the path a gun took to get the gun illegally makes it impossible for the government to stop illegally sold guns. People make the argument that you don't punish the law abiding by taking guns away from them for the acts of the criminals, well you have to compromise somewhere, either make it possible for the officials to see which "law abiding" citizen sold his gun illegally or prepare for the continuance of this assault on the 2nd ammendment.