I've seen it. They think "Pfft, that person is just being picky/dramatic. They won't know the difference."
That happened to me and my X at a Chinese restaurant. My X asked if the Combo she wanted was vegetarian, and the waitress said yes... When the food came, it was obviously covered in meat. When my X said that she couldn't have meat, and she asked if she could just have a new one without it, the waitress kind of shrugged and said "Well... You can just eat around it." We left and I haven't been back since. I don't got time for that game.
It's not the first time that happened, either. Folks get it in their minds that people with eating restrictions are just being difficult.