These explanations are true enough, but somewhat limited.
If you study history, you will find that secular Jews were "progressive" long before they came to America, long before the time of FDR or Senator McCarthy, even long before the time of Karl Marx.
The great, failed revolution of 1848 involved Jews, as did the Russian revolution of 1917 and the brief Hungarian revolution of the 1930s.
Some historians trace this Jewish fascination with worldly progress back to the failed Jewish Messiah, Sabbatai Zevi in the late 18th century, more or less the time of our American Revolution and before the French Revolution. There's a brief article on him here:
http://en.wikipedia.org/wiki/Sabbatai_Zevi
Isaac Bashevis Singer has written some fascinating novels about this period.
When the craving for a religious return failed, it has been hypothesized, many Jews of that time turned instead to the goal of secular progress.
I don't know if that's true or not. But the fact is that Jews have been in the forefront of many progressive movements for more than two centuries, sometimes for the good of humanity, and sometimes not.
Actually Shabtai Tzvi lived in the late 17th century.
The migration of Jews to secularism began with the French Revolution, Napoleon's "New Sanhedrin" and the 19th century Emancipation.