There is no law requiring sick leave or stating how it amy be accrued: companies are free to not have sick leave at all, or if they have it, to set how employees earn it, at what rate. Further, they can change rate of accrual, how many hours or days employees get per year, carry over of hours (whether its allowed, and, if so, how many), etc. at any time, for any reason. Therefore, it is perfectly legal to have the total annual number of hours accrued incrementally per year, 1/12th every month.