Not surprising at all.
One of the seminal books about the folly of the Vietnam War is Neil Sheehan’s A Bright Shining Lie. It describes the career of a former US Army Lieutenant Colonel John Paul Vann who by the late 1960s was working with USAID in the Mekong Delta. The Saigon government was so corrupt that most of the arms the US sent there ended up almost immediately in the hands of the Viet Cong. Vann would plead with his superiors at the State Department to stop sending weapons to the Saigon government, but he was ignored.
Of course, as with all the other crazy parallels between Vietnam and Afghanistan, the same thing happened in the latter country as well (probably still going on). ISIS could have been supplied by friends in Afghanistan, or simply by corrupt officials in the Iraqi government.
The wonder expressed by the media of the discovery of a “rifle manufactured by the Nazis,” a 1941 Mauser k.98, is amusing. This was the standard German infantry rifle during both World Wars, and zillions of them were produced and later sold on world markets, to governments, insurgents and American collectors (like me; I have one, with a plywood — oh, excuse me — laminated stock). Early in the American war with Afghanistan I recall seeing photos of stacks of recovered WWII bolt action rifles such as Mausers, Mosin-Nagants and British SMLEs. I am surprised they found only a single Mauser. It was probably the beloved trophy of some Iraqi townsman or villager who was executed by ISIS.