Employers must take out Public Liability and Employers liability insurance in case they are found to have worked in a way that has caused loss or damage to others. Why is personal responsibility less important? Obesity isn’t always caused by a life style choice, many diabetics become obese. Many lung cancer sufferers never have smoked, we can carry on pointing fingers, substance abuse is dangerous and often terminal.
In the US, people DO take out personal health insurance and personal behaviors DO affect your premiums... believe me, it's not a particularly good system as we've implemented it. I assume the 'suffolk' in your username is in the UK?
Anyway, to take this argument seriously and not just use it as an excuse to demonize cannabis in particular:
How much risk should people be allowed to take before the medical risk is no longer appropriately socialized? Is driving a car OK? Is riding a bicycle? A Motorcycle? Is it OK to eat red meat? Is it OK to consume dairy? Is it OK NOT to consume large quantities of vegetables?
Not eating vegetables is objectively a lot worse for your health than consuming cannabis via non-smoke methods. So if you're going to make people liable for one, you should have a clear stance on both.
Perhaps most importantly, how much surveillance should people be subjected to in order to moderate these risks and who should be in charge of evaluating risk?