You are asking why psychology students are paid while they're still learning, except in Quebec. First, it is true that in other Canadian provinces, there are many more paid internships than there are in Quebec. However, that is not the general situation. It's not true that 100% of the internships throughout the rest of Canada are paid positions, but certainly there are more than in Quebec. When the Canadian Psychological Association grants an institution internship status, it automatically becomes a paid internship, except in Quebec.
I could not tell you why. It is certainly an historical issue. Internships and practicums in the areas of mental health and human relations are not traditionally paid positions in Quebec. However, there are more and more paid internships in psychology in Quebec. But there aren't enough of them and only a minority of internships are still paid. But in universities, there is a movement involving students and professors which is trying to change that.