Why isn't Canada a signatory to the Treaty on the Prohibition of Nuclear Weapons?

The United Nations' 2017 Treaty on the Prohibition of Nuclear Weapons (TPNW) makes it illegally for signatory countries to "develop, test, produce, manufacture, otherwise acquire, possess or stockpile nuclear weapons or other nuclear explosive devices". It doesn't get much clearer than that.

Just this week, Honduras became the 50th country to ratify the pact and thereby make the treaty legally binding on signatory countries. Countries like Austria, Ireland, New Zealand and Mexico have also already ratified it, although most of the other signatories are admittedly small Pacific, South/Central American and Asian nations. The nuclear powers themselves (USA, Russia, China, UK, France, lsrael, India, Pakistan, and North Korea) have not even signed up to the treaty, of course, because ratifying it would require them to destroy their own existing nuclear weapons. There are an estimated 13,000 nuclear weapons in the world, any one of which is capable of killing millions and making vast tracts of the world uninhabitable.

Canada, though, has no nuclear bombs, and there is very little likelihood of it ever doing so. So, you might think that this is just the kind of aspirational internationalist pact that a Liberal-led Canada would be up for joining. And nobody really likes nuclear weapons, do they? But Canada is not a signatory, and indeed was one of just 38 countries that opposed against even holding the conference that led to the establishment of the treaty in the first place, and did not attend the original conference. So, why is Canada against prohibiting nuclear weapons?

Justin Trudeau's usual answer is essentially dismissive and unhelpful: that, if we do not have nuclear weapons, then there is no point in even talking about not having them. Which is a reasonable position at first blush, but there is such a thing as making a point. None of the other 84 signatories and 50 ratifiers have nuclear weapons either, but it didn't stop them from vowing never to acquire them.

When pressed further, the Liberal government claims it just can't join the treaty because it is a member of NATO, and NATO states that "nuclear weapons are a core component of the Alliance's overall capabilities". Most NATO countries, including Canada apparently, seem to believe that US nuclear weapons actually enhance their security, and they have issued a joint statement claiming that the TPNW treaty would be "ineffective in eliminating nuclear weapons", and arguing that full implementation of the Nuclear Non-Proliferation Treaty would be a more effective route (that way, the haves would continue to have, and the have-nots would not be able to have, i.e. ensuring the status quo).

It's notable that none of rhe other NATO countries have signed the nuclear ban treaty either, but there is probably nothing legally preventing them. And what a message that would send.