Well i blame the US for influence this part of the world with their war on drug campaign, and the fear of drug is the direct result of opium war. It's not even that long ago that US started to legalise recreational use of marijuana, it's kinda condescending to it in such simple way.
At least there's been multiple talk by the government of Malaysia (where i'm from) to decriminalise drug, not sure about Singapore.
Yeah. The issue with Singapore is they have extremely strong ruling party and nepotism run strong, so the old perspective will take some time to ease out. If there's more and more evidence piling up on medical use i'm sure they will move to decriminalise.
The only country to have legal recreational marijuana in South East Asia is Thailand, though they're about to pull back the decision.
Not sure it's that simple, since liberty is also a human right, so punishing crimes with prison would also violate human rights if you follow that line of thinking