I would say that atheism (the belief that there is no god) is not organized. But I would also say that the collection of people who want to insist that it means "lack of belief" are becoming increasingly organized. So, there is an organized group that consists exclusively of atheists. I can see where some people might confuse that group with atheism in general.
So why are these people trying to impose a dishonest definition? Not being a mindreader, I can't say for certain. However, I can make an educated guess based on the circumstances in which it first cropped up. The phoney definition was used to say "oh yeah, even babies?" when a negative statement was made about atheists. The existing, standard definition was accepted for positive statements. It would appear that the purpose of the redifinition is to deny a simple term to describe those who believe there is no god (a category people actually want to use at times) to critics and claim that any statement made by critics must be applying to a useless catch-all category of which no general statement can be made about the members.
How do I know they are being dishonest with their definition? The inherently useless nature of catch-all categories is highly suggestive. But these people will slip up. In making positive statements about atheists, they need a term. And that term is "atheist." Their statements cannot meaningfully be applied to infants either.