Focusing in on OP's question, the simple answer is that if one is C19+, one may not fly. Whether there are criminal, civil or internal consequences is a different issue. Thus, in theory there is no person who knows that they are C19+ or displaying symptoms. This does not accommodate those who have not been tested, but are C19+ and asymptomatic nor does it take into account the intentional liars.
Thus, an imperfect solution, but it's all about risk and things that reduce risk are good.