I do think Canada has the right to decide who can emigrate (as I think we do too).
The Globe and Mail: Diabetic prevented from moving to Canada
But Canadian immigration officials rejected the application, on the grounds that his mother, Gizella Szebenyi, has Type 2 diabetes. Mrs. Szebenyi, who was 61 when the application process began, was able to compensate through her diet for the fact that her body does not produce enough insulin. Now 73, she still does not use insulin or any other medication, and has never sought medical treatment.
I do think it is odd that they would accept someone HIV positive before diabetes.