The decision to use the RD-180 for the Atlas V launch vehicle is a result of post-cold war cooperation between U.S. businesses and the Russian defense industry. The RD-180 relationship was urged by the U.S. government as a means of preventing Russian military technology from proliferating. The RD-180 was jointly developed by U.S. and Russian Defense Industries to provide the Atlas V with a technologically advanced and reliable engine. Since then, the Atlas V product has provided enormous value and mission assurance for ULA customers.
http://www.ulalaunch.com/faqs-rd-180.aspx
https://en.wikipedia.org/wiki/RD-180
The reason to use the Russian produced rocket engine was not only to prevent proliferation, but as well financial. The production cost in Russia was, and is, much lower than in the US: The added cost for launches would be over $2 billion,
http://www.forbes.com/sites/lorenthompson/2016/05/23/banning-russian-rocket-engines-will-increase-costs-and-risks/
4/8/16: U.S. needs up to 18 more Russian rocket engines: Pentagon
4/28/16: House panel doubles authorized purchase of Russian rocket engines
Nationalists understand that you NEVER outsource such critical military technology because it creates exactly this kind of national security vulnerability, not to mention the loss of thousands of good paying rocket manufacturing jobs.