Well they did!!!
The Harley Davidson XA.
During World War Two the Army captured a few BMW R71 motorcycles.
This bike was quieter, had a smoother drive system being shaft driven, and a smoother ride because it had a telescopic suspension. The U.S. bikes at this time had none of those features. So the Army had Harley and Indian copy the bikes and build them.
The Harley Davison XA had a boxer which looked very much like the BMW.
Indian produced a wide angle Vee boxer, the Indian 841.
Because the jeep was becoming the go to Military scout vehicle most of the motorcycles shipped to Europe for WWII sat and were not used, many sold off after the war.
For a quick look at the U.S. Military Motorcycles, check out the Yahoo link below.
Motorcycles at war