Formally recognize Atheism as a religion (although one not having a set philosophy of worship, leadership or meeting places), and as such, bound by the laws and limitations affecting all other religions in the 1st Amendment, to "make no law respecting an establishment of religion, or preventing the free exercise thereof".
Atheists petition for laws and policies that do respect (i.e. favor or prefer) their belief in 'no God'. They make their petitions based on freedom 'from' religion, but deny others the freedom 'to' their religion. Atheism seeks to outlaw traditional religion under the guise of freedom.
This equates to nothing more than establishing Atheism as a government favored religion by preventing the free exercise of any other religion.
The 1st Amendment applies to all religions.