According to Larry Kahaner, author of AK-47:The Weapon That Changed The Face Of War, counterfeit production of AK-47s stems from the fact that the inventor of the AK-47 Mikhail Kalashnikov “never patented the rifle’s design – the thought had never occurred to him or anyone else under the Soviet regime.”
Source: Larry Kahaner, AK-47: The Weapon That Changed The Face Of War, (New Jersey: John Wiley and Sons, 2007), pg. 210.