Because Singapore has declared 'War' on drugs. Get caught, you get 24 hours to notify your family where to pick up your corpse. Then they follow through. That's what a 'war' on drugs should be.
The USA doesn't have the stomach for a 'war' so we mess around, spend gazillions of dollars on programs that do not work and drive the profit incentive for drug production and distribution throught he roof. Then stare at each other, scratch our backsides and we wonder why the war on drugs isn't working.
I just got back from Singapore, and the impression left is not that it is an oppressive police state (indeed, unlike American cities, a visible police presence is almost nonexistent), but that people obey the laws because they choose to and want to - they see it as their patriotic duty. Since that philosophy clearly won't ever again take hold in the angrily multicultural USA, legalization is our only hope of limiting (somewhat) the violence.