If they really cared, they'd build some redundancy into their payroll/scheduling.

I really want to know where this is from

That's something we (i study computer science) learned in our 3 months of BWL (Business Management teachings). You got to calculate a rate of people getting sick, and have to higher enough people so that people getting sick is tolerable without things going to shit.

This is a management problem. They don't have enough employees. And don't dare to push yourself to do more work as compensation because someone is sick, unless paid for it.