Any form of government intervention in the markets is out of line with pure capitalism, e.g. farm subsidies; import taxes; bailouts of the auto and finance industries; medicare; medicaid; social security... one could make an argument for Fannie Mae and Freddie Mac as well.

That's not to say that the above are not reasonable policies for an advanced western economy, just not that they could reasonably be considered purely capitalist.

No, there is no "purely capitalist". You confuse "laissez-faire capitalism" with "capitalism". If what you say is true, "laissez-faire" would be a superfluous adjective, like calling it "capitalist capitalism".

There's no American law stating "the US economy must be capitalist." "Capitalism" is just the rough description we assign to the US economy. The principles are there, but there are many exceptions.