For many atheists, our atheism goes beyond the standalone belief that there are no gods. It is a necessary part of this belief that we do not get our morality from gods. This is a significant world view in a society where most people believe the opposite.
That is why there are so many atheist advocacy groups around the world. Ideally, we should not be needed. In the real world, we are needed: to promote secularism in developed democracies, and to protect people in theocratic dictatorships.
I agree with everything in this letter, I hope they succeed in this quest to legalize abortion.