Why would an employer get upset about an employee taking more then one sick day in a row?

I was on quora and asked employers what they would do if someone called in sick on the first day. Most of them said they would be fine with it so long as it was just that one day. Do people not know that someone’s whos sick can be contagious weeks after getting sick, not just after they feel better. Even then, people need more then just one day to feel better from an illness like the flu. Know what im saying? People need more then just one day to recover from an illness.