I don't think that clarifying the meaning of the word "atheism" is pedantry.
The thing is, the word carries a lot of ancillary baggage, especially in certain countries and certain people. For example many people in the United States (some of them not even Christian, or even adherent to any religion) immediately assume that an atheist is also a secular humanist materialist naturalist rational anti-religious immoral strong atheist freethinker who believes in the theory of evolution, the appearance of the universe and life ex nihilo, and who is also most probably a liberal democrat socialist communist pro choice and pro gay marriage. Atheism is also considered to be a movement and faith-based belief system with its own ideologies, doctrines and goals. That's quite a lot of baggage to carry.
Getting rid of all those assumptions that the word "atheism" does not convey is not pedantry.