Because Quebec. It's my home province and I kinda like it but some things are so backwards and retarded. Weed for instance. It's legal throughout Canada but Quebec had to make laws prohibiting smoking it on public streets and parks as well as apartments. So it's technically legal, but you can't smoke it anywhere.
The landlord can put a clause prohibiting cannabis smoking in the rental unit which is the case where I live. In BC, if there are is no clause prohibiting the smoking of cigarettes then logically,it applies to weed to. Weirdly, there is no clause prohibiting cigarette smoking in my building but weed is strictly forbidden as per the lease which doesn't make any sense at all. So I indulge regardless. As long as I'm not bothering other tenants with the smell it's all good.