Well, yes. As far as immigration goes, the policy of most non-US western countries is primarily merit-based or if you have money to invest in the country. If you aren't educated and productive and you're not a legitimate refugee, why should a country let you in? How does the country benefit?

Canada has a generous welfare system and social safety net that that would likely be unsustainable if it let in sufficient number of people unable to support their own benefits. Even if you hold the view that drug use ought to be a public health matter and not a criminal matter, there's a limited amount of immigration that can be sustained without overburdening these services and why let in a drug user when you can let in a doctor or engineer?

In the case of tourism, it's just about limiting risk of someone overstaying their visa.