If a life-insurance company had evidence that a person were an alcoholic, you can bet they would either deny them coverage, or raise their rates through the roof. A life insurance company wants to avoid paying out more than it has too. It has to, or it would not be able to stay in business. I suspect that they have some sort of metric that show that people with more than a certain amount of marijuana in their system is more likely to do something stupid that results in death, or they are more likely to die of other causes. It may also be a way to screen out people using Marijuana for pain relief, which would indicate the possibility of a serious health issue.
I suspect that cigarette smokers generally get really high rates, low payout amounts, or outright denial, depending on the risk that the life insurance company thinks they will be.