While that may be true, it is also true that the environment is shaped by the people that inhabit it. Most of the people in the US come from immigrants, people who weren't afraid to set out on their own to a new place. Those kinds of people generally value individual freedoms over the stability of society.
I'm curious what the actual stats are with regards to that theory. More than a few historic immigration waves were prompted because things were so bad "back home" - for a variety of reasons including social instability - that the fear of setting out to a new place was far less than the fear of remaining. I'm not sure if that was so much an issue of individual freedom.
Examples off the top of my head would be the rise of the communist party in China and the Irish potato famine.
However, the first wave of Europeans colonists to come the US definitely did value individual freedoms over stability. I don't know how strong their influence is on the population itself, but their views are definitely reflected in the laws and historical document (e.g. The Constitution).
The short answer is that the data doesn't support generalizing the motives of American immigrants over 300 years from very diverse social, political, and economic conditions. There is no suitable generalization. The same goes for attempting to describe America's current cultural values as a single group.
Longer answer requires the gradiated initial cultural values held by every significant immigrant group (puritans, slaves, Irish, Chinese... significant defined by impact, population, whathaveyou), determining how resilient those values were when thrown into America's melting pots (assimilated? insular?), and to what extent they influenced the groups around them over time. That's a career question, though - not a HN comment that I'm underequipped to answer.
Even in the case of migration to avoid a bad situation, it's still a small minority that left whatever the old place was. The vast majority stayed there, so you've still got a tiny self-selecting population of immigrants.
