Atheism, Catholism and numbers

Reading America's Atheists, Believe it or not, the fact that surprised me wasn't that 30 million Americans claimed "no religion," but rather that Catholicism is America's largest Christian denomination.  Growing up Catholic, I never felt like I was in the majority (except when we lived in Spain,) and I'm still surprised to find out that friends are Catholic.  I wonder why that is?

The same article also said the United States' problem with religion is:

• over-aggressive proselytising in the armed forces,
• undermining science or AIDS programmes,
• alienating minorities at home and Muslims abroad

and suggested that if America's atheists were more politically active they could help fix those problems.