Private health care isn't illegal. What it is, or can be, is a violation of the Canada Health Act. The Canada Health Act is not a law in the sense that it makes things illegal. It is a funding agreement between the provinces and Ottawa. Each province and Ottawa voluntarily agree to it, and can drop out at any time. Basically, it says that Ottawa will provide transfer payments to each province if that province abides by the principles of the Act. One of those principles is not allowing private billing for insured services. Insured services are defined as medically required hospital and physician services. So, things like physiotherapy, optometry, etc. are NOT covered by the Act, though some provinces choose to cover all or part of those services as an extra benefit to citizens,, or at least certain citizens such as seniors and children. Anyway, if a province allows private payment for insured services, it can be deemed to be in violation of the Act. The penalty for violation, as written in the Act, is that Ottawa can withhold transfer payments to the offending provinces, at a maximum of $1 for every $1 in private billing allowed.
Ottawa chooses to turn a blind eye to much private health care, even if it a clear violation of the Act. An excellent example is the Cambie Clinic in Vancouver, that performs medically necessary services and charges the patient directly. As far as I know, this has never been declared a violation, with penalties imposed.
I suspect the main reason that Ottawa ignores such violations is two-fold: first, the Canada Health Act was predicated on the assumption that Ottawa would pay 50% of the costs of the health system, with the provinces picking up the other 50%. However, under the Chretien administration, the federal portion dwindled to approx. 14%, with the provinces picking up the rest. This is probably the biggest single reason why all provinces are struggling to fund health care. This diminished federal portion also diminishes the feds' moral high ground when it comes to lecturing the provinces on health policy (though it never stopped Jean Chretien from proclaiming his government the savior of health care). The other possible reason for Ottawa's blind eye on violations is likely ideological; many in the Harper government favor the private health care option so aren't about to start penalizing provinces that allow it.