Atheism does make a positive claim: it makes the claim that God doesn’t exist.
There is a difference between lacking a belief in something, and positively believing that something doesn’t exist. Atheism is the latter, while non-theism is the former.
Babies are not atheists because they don’t actively deny the existence of God; they just don’t have an opinion on the matter at all. Now, God is often compared by atheists to unicorns. Most of us actually positively believe that unicorns don’t exist. I think it is childish wordplay to say “I only lack a belief in unicorns; I don’t have a belief that they don’t exist.” When we talk to kids about unicorns, we say “unicorns aren’t real.” We don’t say “I don’t possess a belief in unicorns.”
Atheists, like everybody else, have a viewpoint. They have an opinion. It’s really a simple issue which I think a lot of people try make more complicated than it is.