Theoretically, our Bill of Rights says that the government cannot establish a state church, and force citizens to attend or adhere to the state religion. The Constitution blocks government intervention in religious matters.
Obviously, this is not well-enforced.
But the "Freedom FROM Religion" foundation is a private organization which is attempting to get the government to stop people from praying.
This is not a question of "we not enforcing the Constitution". This is a war against the practice of religion. The Constitution does not sanction such a war.
It blocks the national government. Several states collected taxes into the 19th century on behalf of certain sects.