I am an American who has lived in Thailand for several years.
The US has a huge drug problem and SE Asia doesn't even come close. The local population does not abuse drugs at all. Nearly all of the drug trade caters to American/European tourists. Which is incidentally why the punishment is so harsh, they do not tolerate foreigners poisoning their society.
Enforcement works.
I feel like if crazy penalties like that were levied in America we would take it as a challenge. We have a long history of that.
If anything, stigmatizing a situation only drives it underground where it isn't talked about, and essentially flourishes in the darkness.
In this case, how would foreigners be responsible for "poisoning their society"?
To me this sounds a lot like what you here in the West concerning drugs... America claims its' foreign traders with "terrorist connections" who do drug sales (e.g. all the talk about Colombian drug cartels, or Mexican drug wars).