You read why they got convicted right?...
-Erasing serial numbers.
-Concealing full auto guns.
-Missing guns.
So what would be a stupid reason?
If these guns were on their way to Mexican drug cartels, there would have been no problem. (Google Operation Fast and Furious).
On another note, there were no serial #'s until many years ago, and the US Constitution does not authorize the government to mandate serial #'s, nor SBRs, and many other things.