One gal even made me take off the mask and put on a surgical mask (which she gave me, she wouldn't let me wear my non-vented N95) in front of a security camera. That was pretty embarrassing, to be holding up the line, and felt like a scene out of a dystopian comedy.
I can only reason that it's for liability reasons, and the airlines don't want to be sued for someone getting infected.
But then again, people right behind me had cloth masks on with their noses exposed, who obviously didn't care, and they didn't get a second look. Neither did the airlines keep the middle seats free like they had promised too. Clown world.
Once I was on the flight I put my respirator on with a hoodie over it and slouched against the window, and they didn't bother me.
A friend of mine postulated that they couldn't hear me through my mask, but as I took it off and was swapping I said "I'm complying, I'm complying, I'm not arguing" in a conciliatory tone, it didn't seem to help. I was playing the 100% doormat. Didn't stop the attendant from saying to me as I was leaving to go to my seat: "It's not that dangerous".
I can only guess that they encounter people who do insist or that they are on web forums / chat groups with other attendants who have encountered people who do argue and so they're primed for someone like that. Puzzling interaction, to be honest. It was the first time I've had any sort of negative interaction with a service professional.