How does the author describe communal voluntarism?

From the text:

The answer, I think, is that Americans have historically adhered to a distinctive view of the individual’s place in society, a view that can be called “communal voluntarism.”
Americans insist on the reality and value of individual free choice, including, critically, free choice to join or leave groups, be they companies or countries. However, Americans also believe that, once individuals are members of the group, they must be loyal. You could think of this philosophy as “love it or leave it” — with the understanding that you aren’t forced to join and are genuinely free to leave.

Americans believe in contracts — or covenants, to use religious language. Our culture insists that if you marry, if you take a job, if you join a club, and so on, you are signing an explicit or implicit contract to cooperate and conform. If the group no longer works for you, the door is open. American-style individualism lies in the freedom to choose; American-style collectivism lies in the commitment to the group that freely choosing entails.