If someone gets sick, then you either wind up short handed or call someone and ask if they can cover the shift. That's not the problem here, though. This whole discussion is based upon employers asserting that they cannot schedule more than a day or two in advance, that it is an undue burden upon them to schedule employees to consistent and regular shifts, and that it is employees rather than employers who should have to suffer the consequences.
That's complete and utter bullshit. Again, I've been in this situation before as a manager. You don't just throw up your hands and refuse to make a schedule just because one of your line cooks called in sick one day. You make the schedule based upon projections and adapt if necessary. If things are slow and you send a guy home early, then there's a sudden rush, guess who gets to put on an apron and help out in the kitchen? The manager. Because if I called someone and said "hey, looks like we've got an unexpected late afternoon rush" they'd probably arrive to late to do anything except get pissed off and go back home.