The Germans SS soldiers dressed in US uniforms had their own uniforms under the US uniforms. They were ordered to magically shed their US uniforms before they engaged in combat. Of course they didn't in some cases.
The people who paid for that little act of chicanery, and the murders at Malmedy were the German soldiers who were not captured. US soldiers in WWII often didn't accept the surrender of German soliders who had serrated bayonets, as such weapons were thought to be intended to inflict unnecessary pain and suffering. After this became known, German soldiers so equiped would often drop them on their way to assault the US positions, or on their way to the front lines. Some germans ground the serrations off the bayonets.
An incentive to follow the rules.