One possibility: old habits (such as working in spite of being ill, not wearing a mask while ill) die hard, even in the face of a serious pandemic. Another possibility: it’s allergy season, and they know they’re having seasonal allergies. In a sense, it’s their choice. But ethically, their choice should be influenced by its potential affects on others. Daily, we avoid doing things (like smoking indoors or driving drunk) that could harm others’ health.