As a sociologist -and an atheist, if you must know- currently researching the stigma that atheists experience, I applaud PZ's comments.
Why do atheists speak out? Many reasons; we are not a monolithic group of people. As PZ points out, we are everywhere in American society and represent a wide array of viewpoints, perspectives and levels of activism.
I conclude from my data set (n = 8204 atheists) that many, many atheists are in the closet almost completely, fearing the social stigma that comes with the label "atheist." Why not come out? Many that have come out report losing their jobs, their friends and associates and even the connection to some family members. Those that chose to stay in the closet fear these consequences. They will continue to live these lives of fear until atheism is destigmatized in our culture (like it is in the UK and other Western nations).
Many of us are determined to fight the fight of destroying the marginalization of atheists. We will continue to speak out.