IIN I get annoyed when atheist try to push atheism on other people?

I know this is off topic, or a tangent at best, but what's with people using "atheist" as a collective noun?

All I can think of is that "ists" is a slightly awkward phoneme. But I don't see people making the same mistake with say "feminist(s)". Nothing about the word "atheist" suggests it should be plural, as far as I can tell, yet I see people treating it like it is all the time.