One of the benefits of working under a contract is the ability to call-in sick without having to worry about recourse.
Some of the worst colds on earth (you know, the ones that you thought you'd be better off dead) were ones that I caught from pilots and flight attendants that were flying ill to avoid having to call in sick.
Now if a crewmember is ill and doens't want to get a nasty 'tick' on his/her employee file that'll be under circumspection every five years, that'd just going to put everyone else in a precarious situation.
Or even if you've got to refuse a jet that is airworthy, but not legal to fly, what's a boy to do?