This is a piece of idiocy by governments who want to control their population. The US spend billions of dollars a year trying to stop drug consumption by their own citizens. The parties which supply drugs are being financed by US citizens, and the government fights against it.
The people of the US who take drugs pay for the war twice. They fund the drug suppliers and the law enforcement agencies who try to prevent supply. The people of the US who don't take drugs pay only once. They pay for the law enforcement agencies to try to stop their drugs.
Clearly the US would suffer a productivity loss if drugs were legal. A large proportion of their (more stupid) population would just vegetate in front of TVs or lava lamps. But this would be less than the cost of funding the jails, law enforcement agencies, and the crimes which result due to the high price of drugs. However the society drop-out rate would also lower the government's tax revenue. People can live very cheaply (and hence pay very little tax) if they are prepared to spend 90% of their lives sedated, and many would do so.
The US, and many other first world countries, are fighting a war with their own people, but it is one they cannot win. All civilized countries have realized that alcohol prohibition did not work. How long will it be before they realize the same of other drugs?