Funny that Layton was offered such easy terms. How often does the government "win" a case at the Supreme Court and then offer a plea-bargain? Methinks the government really lost, but was so successful at their "spin" they were allowed to claim victory.
The "Government, that is the Justice department doesn't get to set the punishment. Since the original judge thought the law was no law at all, one would expect him to set the minimum sentence he could, given that the defendant entered a guilty plea.
The government didn't really want to put Layton away, they wanted the law held to be Constitutional. In that they won, in a very limited sense. But you are correct, the gun grabbers have spun the ruling into way more than it really was. It's also the last time the Supreme Court directly ruled on the Second Amendment. They now avoid doing so like the plague.