Well, it seems the Canadian Federal Law makes the PIC a law enforcement officer. Doesn't seem to say anything about requiring or authorizing being armed. Do Canadian pilots receive training in the law or use of law enforcement authority? Trying to imagine that as part of the written test for PPL.
In the U S context, the postal inspectors can be thought of as a Secret Service/U S Marshalls Service with juristiction over the U S mails. These people are the real deal as law enforcement people, so comments about armed "postal workers" are either humorous (?) the first few times or purposefully thick. When air mail started in the U S the pilots were required to wear a sidearm so that if they had to land somewhere and let the mail continue by other means they could guard it until it's custody could be passed. Look at some of the old pictures of airmail pilots and you can see the .38 revolver or some such in many cases.
Qualified armed people would be okay to have aboard in theory, but from a practical point of view it is difficult to establsh the bonafides of people from a zillion different agencies and it would be a good idea to limit it to people with proper training (done in U S to some extent) and properly preflight briefed. PIC permission should be required, another item on the preflight checklist. Just make sure they don't shoot themselves in the backside when they clear their weapons.