Remember your world history. The East India Company was a publicly held
stock by merchants and aristocrats. They were completely unrelated to the crown. That didn't change the fact that they were the de facto rulers of India (complete with a standing army and navy)
http://en.wikipedia.org/wiki/East_India_Company
Companies can become Governments rather quickly. The difference between Companies and Governments is that there is none. The US Government is simply one of the largest companies created on this land.
The US Government has also claimed monopoly privileges on armies and police force. Within the lands that the US Government owns, no other groups of people can organize an army. Basically, Companies and Governments are all the same thing. They are a collection of people, attempting to work together for some abstract purpose.
Besides, the US Government has adopted anarcho-capitalism before. Calvin Coolidge's quote: "Perhaps one of the most important accomplishments of my administration has been minding my own business."
I'd say, Calvin Coolidge probably did more for the Anarcho-capitalist cause than any thought experiment revolving around an Anarcho-capitalist utopia. A heavy pusher of deregulation and a "Hands Off" approach to governing.
Certainly, Calvin Coolidge was less oppresive to the American population than the East India Trading Company was to the Indians.