Not a law, but a case of supply and demand. In the 50s & 60s, WWII wasn't that far back in time. The whole world was flooded with milsurp rifles, especially bolt action. The war was over, and not nearly so many rifles needed as while most of the world was at war. At the same time, semi-auto or full auto was the only military rifle governments worldwide wanted, so they dumped all the bolt action rifles. Milsurp bolt guns had very little value. They were everywhere for next to nothing. It was about a $10 Carcano that Oswald used. So everyone added value to them by "sporterizing" them, my Dad included. Today, the very nicely sporterized bolt rifles have some value, but original condition bolt milsurps fetch considerably more money, because they're now relatively rare.