The federal govt does not own any companies. Many companies however are contractors to the US Govt.

Since I assume you are a SA citizen most likely this is not a military or intelligence position, which sometimes are subject to drug testing. In my experience for contractors rarely does any drug testing occur even for classified positions. If it happens at all it will be at the very beginning. The US government for contractors typically requires a "drug free" workplace which does not mandate testing.

As others have noted you simply should ask HR what the Drug Testing policy is. I would probably ask the HR person and not the hiring manager (the person you'll actually work for).

Generally speaking very few white collar jobs require random ongoing drug testing unless you work in the medical industry or a blue collar job where you operate heavy equipment or construction.

If you think about it, piss testing is most likely only catch marijuana anyways. Most companies can terminate you at will for performance so what's even the point.