All the more extraordinary because Mr Shaw's party is one of the leading lights in the drive to make New Zealand tobacco-free by 2025. (Presumably the term smoke-free is now redundant).
If all goes according to his plan, a substance that harms the physical health of the user will disappear just in time to be replaced by another substance that does even more damage, physically, emotionally and intellectually, than tobacco ever has.
We know that cannabis is a carcinogenic, as is tobacco. Unlike tobacco, however, it is also linked, beyond dispute, with mental illness and poor academic achievement. From there it can be held accountable for reducing the user's ability to find employment, and everything that goes with that, including poverty, for themselves and their dependents.
The drive for legalisation has taken a turn (for the worse) this time around because of strident appeals to recognise its medicinal benefits. It might well dull pain - it certainly dulls most of the user's senses - but there is a undoubtedly deliberate blurring of the lines by the drug's supporters between medicinal cannabis, which does not include its mind-altering properties, and the 'benefits' to be gained by allowing its cultivation/possession and consumption in the traditional manner.
People have long waxed eloquent about cannabis as a pain killer, usually from the dock as they are in the process of being sentenced for growing the stuff. If personal experience of that is anything to go by, its fans tend to show all the signs of long-term use, which might make them happy but has reduced their role in society to that of passengers.
It might well be true that cannabis does not represent any great threat to the physical or mental health of a middle-aged dope smoker who indulges on an occasional basis. The same cannot be said for those who start young, and there, Mr Shaw, lies the rub.
We have been told for years, most often by the National Organisation for the Reform of Marijuana Laws (Norml - there's an oxymoron for you) that legalisation would of course need to be accompanied by strict controls that would keep it out of the hands of young people.
That assurance has been given to the writer on numerous occasions, but no one has ever been able to explain how any such measures would stand any chance of success, given our experience with tobacco and alcohol.
Neither of those substances may be legally purchased or used by minors, but both are. No one in this country has yet been able to devise controls that prevent that, and the same, inevitably, will apply to cannabis. Prove to us that you have cracked that, Mr Shaw, and people might start listening to you.
The best reason for not legalising cannabis was offered to this newspaper some years ago by a teacher at Kaitaia College. He said the college was home to any number of bright, determined, ambitious young people who knew what they wanted to do with their lives, and had mapped out exactly how they were going to achieve their ambitions.
They knew that even a minor cannabis conviction would nobble those ambitions, and for that reason alone wouldn't touch the stuff with a barge pole.
No one the writer knows has ever come up with a better reason for not legalising it. And no one will. If it is legalised future generations of bright, ambitious young people will assuredly dabble in it, to their (and our) cost.
Even if they don't succumb to regular use it will rob them, to some degree, of their potential, to a far greater degree than flirting with alcohol or tobacco ever would.
We don't hear Mr Shaw, or anyone else, suggesting that our children should have greater access than they already do to alcohol and tobacco, for good reason. How they can be prepared to countenance access to cannabis defies explanation.
Perhaps Mr Shaw's political interest in this issue outweighs any concern he might have for future generations. Perhaps the legalising of cannabis has such appeal to his voter base that he can accept the inevitable collateral damage. Hopefully he is in a very small minority, and will remain so.
And don't buy the hoary old story that our prisons are full of people who wouldn't be there if cannabis was legal. Those who insist that this is true have either been doing too much personal research into the 'benefits' of sucking on cannabis cigarette all day or are deliberately trying to deceive.
No one is in jail in this country today purely because they have been caught using cannabis. One or two might be there because they were caught growing or dealing it on a substantial scale, but possession of cannabis, whatever the law might say, is no longer an imprisonable offence in this country, and hasn't been for a very long time.
There will be some who are in jail on convictions that include possession of cannabis, but it won't have been the drug that put them behind bars. They will have offended in other ways. To say that people are in jail because of personal possession is a blatant lie.
Some elements of the current debate are certainly worth pursuing, including that drug addiction in general should be regarded as a health issue rather than a criminal matter. And there is no doubt that drug treatment facilities are woefully inadequate. But again, this is where the pro-cannabis logic collapses.
We know the harm cannabis does; we know it leads to dependence on much harsher chemical substances; we know that people who become addicted, to whatever substance, are unlikely to get the help they need to get off it. And we know that the damage done, by cannabis and other drugs, is permanent. Dead brain cells don't grow back.
Yet here we are talking about legalising it. It makes no sense whatsoever to even consider it. A handful of people might genuinely believe that it will ease their pain, or, in medical form, will reduce the severity of some far from common conditions (again, the use of medical marijuana is a separate issue), but legalising cannabis for all and sundry will not benefit society in any imaginable way.
There can be absolutely no question that legalising cannabis will, in fact, do enormous harm, and any politician who is unaware of that, or is prepared to trade that harm for electoral success, has no place in Parliament.
