How come athesists say they do not have a religion but all the books on atheism?

Atheism is a topic within the overall subject of religion. Look at the rest of the books in that section - do you really think they're all religions?

A book on a subject does not automatically make the subject a religion.

Now, there are some religions out there that are atheistic but not all atheists belong to those religions. In fact, most atheists do not belong to any religion. The words atheist and theist are not religions. These are merely stances on whether or not a god exists.

I believe the Supreme Court ruled Atheism is a religion.

Theirs is called The Book of Pages. Do a yahoo search. Also Google Toynbee Tiles.

I lack belief in GODS. Gods are part of religion. It's not a culinary decision, a sexual orientation or a hairstyle. It's a religious opinion.

Atheism is a position on Theism and so is put in the religion section...

That's just marketing, plain and simple. Two types of people will buy a book on atheism; people who question their faith and people of faith who want to understand and/or refute people without it.

Atheism can't be a religion, by definition.

Atheism isn't officially classed as a religion, however, it is considered a "Belief system" or an "Ideology", and belief systems & ideologies are often put in the "Religion" category.

Because it is a position on the subject of religion.

Atheism is a religious topic, but is not itself religion.