Being privately owned they don't have to.
I'm not sure about that. Private entities can't, say, fire you for your religious beliefs. So, I'm not sure about the legality of them deleting a group for, essentially, a religious belief.
And, while I don't want or care to debate "is atheism a religion", the point is, for people who actually get it and don't want to just flex their e-penii debating semantics, is that athiesm is a perspective on religion. So, in that sense, I'd think, for example, you can't fire someone for being atheist.
Now, these are employees, its not that type of relationship. But, can, say, a hotel refuse to rent rooms to people based on their religious convictions? That might be a more apt comparison.
Oh, and didn't the OP mention it was agnostics also? Agnostics actually do profess a belief in a higher power.