"Another advantage government employees enjoy is that unused sick days add up, which can be a nice departure bonus."
There are two types of federal employees. Those hired before 1986 and those afterwards. Those hired before can use their sick leave days as you suggested. Those hired afterwards lose all unused sick leave at retirement. While that may or may not right, the net effect is that in their last year or two prior to retirement, those scheduled to lose their accumulates sick leave are sick a lot.
It's a darned good way to lose your pension too since they'll haul you into federal court and punish you under the criminal code.