It depends whether you want to be scientifically correct. The job of a scientist is to analytically separate different effects that may have similar outcomes. The overall level of political freedom can be produced simultaneously by several different factors, such as: - the quality of political institutions; - the overall level of economic prosperity; and - the absence of any serious challenge to the status quo.
The US has a relatively free political system mainly to the two latter factors. It is a rich country with rather low population density, which has more to share and more room for assorted misfits, and that tends to keep most people relatively content and not interested too much about politics. Moreover, for all practical purposes any meaningful opposition to the status quo has been effectively neutralized while preserving the facades of democracy.
As a result of these two factors, the actual margin of political freedom in this country is relatively wide at the moment, even though its politcal institutions are among most undemocratic in the world (save for known dictatorships, theocracies, and quasi-fedual monarchies). However, if any of these two conditions changed, the ruling class would not hesitate to throw all constitutional guarantees through the window and resort to outright repression - the treatment of communists and labor organizers is a case in point.
wojtek