They fight for
women's reproductive rights, among others. :)
She was a customer, not an employee, but... no, this is not reasonable. If a co-worker overhears a conversation that offends them when that conversation is not antagonistic in any way, disciplining the conversing employees would be absolutely insane.
Of course they should, because that's openly bigoted, hostile and antagonizing to them and that would create a hostile work environment. A co-worker asking another if they believe in God and them discussing the answer to that question is not an attack on anyone.
How is it confrontational? Atheism just means a lack of belief in gods. That's it.
That people would seriously suggest, or even claim, that the word "atheist" is confrontational, is absolutely mind-blowing to me. I could kinda see it for calling yourself a Satanist, but atheist? Not even a little bit. It's as neutral a word as it gets. Calling yourself an atheist is as "confrontational" as calling yourself a Christian.