How can Atheists offer prayers to the Unconquered Sun?

“At this season of the Winter Solstice, we celebrate the birth of the Unconquered Sun — the TRUE reason for the season. As Americans, let us also honor the birth of our Bill of Rights, which reminds us there can be no freedom OF religion, without having freedom FROM religion in government.”

This is the banner at an Atheist display set up to balance a nativity scene.

Isn’t worship of the sun just as much a religious act as worship of a baby in a manger? Wouldn’t a true atheist not worship either?