Atheism acts LIKE a religion if people organize, hold regular meetings, creat websites and publish documents promoting it in order to 'sell' the idea.... that is, anti-theistic proselytizing.
This is a silly, useless activity in my opinion,,,but it is still basically protected as 'free speech'.

Just replying "I guess I'm an atheist." when asked does NOT make it a religion, and that court may have had a point if a group was trying to form an organization in prison specifically to tout their DISbelief....not a good point, but if someone acts like many religious groups do, lines get fuzzy.

Most atheists do not 'join' anything, and the word is just shorthand for "I don't accept all the stuff churches tell me."