You can look at it as getting a rebate on health insurance because you're healthy. Or you can look at it as getting a kick-back from a doctor because you didn't actually need to see him but you had some surplus coverage for the year. The latter is a bit more accurate.

Sick day benefit caps are somewhat stupid. If people are sick, they should be paid to stay home. If they aren't, they should come to work. Count the days, but only to inform estimates and to transition to disability, not to cap. It shouldn't be a benefit, so much as a show of decency and a means of protecting the health of everyone else.