The U.S. Army is buying Russian designed 7.62mm PKM machine-guns from Hungarian firm Vltor Weapon Systems. The PKM is one of the most widely used machine-guns in the world, and the U.S. Army uses them to familiarize American troops with this weapon, and for OPFOR (Opposing Force) training. This is when there are military exercises in which American troops are opposed by soldiers dressed and equipped like a potential enemy. Since most current, or potential, enemies of the United States use the PKM machine-gun, you need a lot of them in stock for training purposes. The Special Forces will also need enemy weapons for some types of missions.
Why not buy PKMs from the Russians? Mainly because Vltor manufactures a sturdier version of the weapon, that is less likely to break, and will last longer. That saves money in terms of lost training time, not to mention aggravation. There are many non-Russian companies making Russian designed weapons. Many of these are cheap knock-offs, but some firms find a market by making high-quality versions for more discerning customers.