The word "Jews" can refer to a distinct ethnic grouping or to the followers of the religion of Judaism, pumpkin. A person can be the descendant of ethnic Jews yet not believe in any gods - an atheist Jew.
A Muslim is, by definition, a follower of the religion of Islam, in much the same way that a Christian is a follower of the religion of Christianity. You have to make a conscious declaration that you belong to those religions - I've never heard of anyone described as an "atheist Muslim," and there is no ethnic group called "Muslim" or "Christian." It's possible that a person could be an atheist, but due to the sometimes extreme disapproval with which atheism is viewed in Islam, might continue to practice Islam to cover up their atheism. I'm sure that happens. Is that what you're talking about? Because there are atheists who do that with Christianity, too.
I have also seen people refer to themselves as "cultural Muslims" or "cultural Christians," or people who don't necessarily believe in the religion but continue to participate in the culture - holidays and rituals and so on. Maybe that's what the "atheist Muslim" you mention really was...and if so, there are "cultural Christians," too.
And there's I?ISLAM, telling Christians they're following their religion wrong again because they're not following it like Muslims (aka members of the 7th Century Re-enactment Society) would do, which is like telling baseball players that they're wrong because they're not playing by the rules of golf.
How tedious.