Dick,
Civil Air have proposed a cap on single day absences which Airservices have declined. Airservices propose 15 days a year, 5 without a Medical certificate, and unlimited paid sick leave for absences longer than 28 days. What happens if you are sick for 21 days isn't mentioned.
I'm not exactly sure how the current conditions were originally put in place, but as they have been around 15 years at least that I know of, I would suggest that it had something to do with the fact that former regulatory bodies (DCA, DoT, DA, DoTC, CAA) had, as part of their charter, responsiblilty to the
Regulations. This is not the policy of airlines or government-owned corporations..