It’s not clear at all to me why the Governor General cannot be called Canada’s head of state in some contexts.
(Monarchists please direct your hate mail to my correspondence secretary, Nigel. Please use a fountain pen, or if you prefer, quill.)
Not that I’m claiming to be an expert on matters vice-regal. In fact, all I know for sure about Rideau Hall is that when a cricket match is being played on the grounds, it’s good manners to start your tea with the cucumber sandwichs, but if they’re playing crochet, etiquette and tradition demand that you begin with cheddar and chutney on a triangle of crustless bread.
Be that as it may, in 1947, King George VI signed so-called “letters patent,” which gave the Canadian GG the main duties of a ceremonial head of state. And starting in Governor General Roland Michener’s time, GGs traveling abroad on Canada’s behalf began to be greeted by foreign governments as Canadian head of state.
So the GG has the duties of a ceremonial head of state. The GG gets treated like a ceremonial head of state. If it looks like a duck, and quacks like a duck, and wears a lot of brocade on its epaulets…