Actually, the Ontario government has military leave entrenched in legislation (well, technically, in regulations). The General Regulation (R.R.O. 977, 1990) to the Ontario Public Service Act specifies, in section 67:
A deputy minister may grant leave of absence for not more than one week with pay and not more than one week without pay in a year to an employee in his or her ministry for the purpose of Canadian Forces Reserve training.
So the OPSEU CA is simply recognizing what is required by law (I know it says "may grant". However, in application, a DM would have to have justification for not granting the leave, or it will just result in a grievance. For example, someone in a key position with no available backfill might be reasonably denied, but just denying it on a whim is going to have it end up in front of the Grievance Resolution Board, for no good purpose.) Moreover, based on the same Regulation (section 70) the Ontario government can grant substantial periods of discretionary leave without pay--generally, up to two years, but with adequate approvals, up to five years. Notably, Bill Davis, when he was Premier, signed a policy specifying that military leave was a justifiable reason for granting up to five years of leave (this is a policy, mind you, which, unlike regs or legislation, is relatively easy to change. But it hasn't been since Davis enacted it). As a result, I had no difficulty getting a year of leave to deploy to Bosnia. But even with all this legislative support, my boss was excellent about it, so much so I nominated him for CFLC award.
I hasten to add that I fully acknowledge that few Canadian employees who are Reservists, especially in the private sector, enjoy this sort of leave support. I absolutely wish it was otherwise.