It all comes down to politics. Were politics not infused with religious ideology, I would not give so much as a single shit what any individual's personal theology was. We could live, and let live. I like blue, you like red, mix nox.
However, when governments create, and are encouraged to create, laws that incorporate the religious views of one group of citizens into the enforced policies that affect the entire population, then it becomes a concern for me, and other atheists. Now, your beliefs affect me. Government creates a problem when it imposes, through enforced laws, your religious positions on me. I'm certain that Christian, Muslim, or other theists would very much understand this position, were they to suddenly find themselves in the minority in their home country, and were suddenly forced, by virtue of government laws, to behave in accordance with a religion to which they did not ascribe.
The United States, in spite of the First Amendment designed to separate Church and state, is a highly religious country, ruled by increasingly aggressive religious political leaders. The globe is littered with examples of failed attempts to incorporate religion with governance. My concern is that one day America might be counted among them.
It is for this reason that, as an atheist, the beliefs, and actions of theists concern me.