I read a 33-page British paper from 1995 so you don't have to. (It's the first result for "why is japan anti drug" on Google, can't get the right link to it.)

Summary:

• Japan's drug problem seems inconsequential to Westerners who see "urban decay, drive-by shootings, crack-addicted babies, and gang warfare", as none exist in Japan. However, many Japanese people still believe it is a serious issue.

• Pre-war (before 1931) Japan was aware opium was used to undermine China and was already beginning to ban its use, importation, and instituted public education against it. Opium use was "not a problem in Meiji Japan".

• Drug traffickers were able to make a profit in Europe/Americas -- they could not in Japan. (It also mentions cultural values later, though I'll try to summarize more materially. I assume you can draw your own conclusions what these economic conditions would mean ideologically.)

• In the war period (1931-1945): "According to the League of Nations, 'one of Japan's first official acts following the formation of the puppet state of Manchoukuo was the creation of an opium monopoly which not only produced and sold opium but also manufactured derivative.'" Japan banned drugs at home while encouraging drug trafficking in the nations they occupied, to the point of manufacturing heroin/opium themselves.

Damn, it do be like that for capitalist states tho.

• 1940s/50s sees an economic depression and subsequent rise in drug use, along with stimulants (a vestige from the war) to increase factory productivity. Japan's drug laws mirrored America's by this time, although stimulants were not covered because America had no personal issue with stimulants. Later laws ban stimulants, however.

• "Stimulants acquired such a bad reputation in the Japanese press that major drug dealers started to traffic narcotics instead." Early 1960s see methaqualone use rise. (I think it's a sedative, I dunno.)

• Kurosawa's great film "High & Low" is released and has an excellent scene of the villain injecting a woman with heroine to test its purity, and the reflection of his sunglasses shows her dying. That's not in the paper, I just got bored.

• Post-1960s sees stimulant arrests rise dramatically again. It also mentions the economy boom that leads to many young students rising in to the middle-class and now having disposable incomes for weed. I thought of Yakuza 0 where enemies are so rich money flies out of them when you punch them. Also, hippies.

• Holy fuck there's a ridiculous amount of stimulants (legal/otherwise) on the Japanese market. No wonder they have coffee-in-a-can. Then again, we had 4Loko.

• Cultural stuff: public education doesn't seem to reach that drug use is linked to material conditions. More Japanese than Westerners believe drug users are merely evil or have poor self-control, thus greater punitive measures. The Reagan administration also economically pressured Japan to join the "war on drugs", and Japan's punishments are the most severe in Asia.

• Japanese police are also not allowed to use telephone wiretaps (didn't they use one in High & Low?), have weak subpoena power, and do not have the capacity to run undercover operations. Money laundering was also not initially illegal in Japan. (I wonder if the punitiveness has to do with that; needing harsher punishments, after which comes justification, after which harsher anti-drug attitudes filter to the populace who will not be terribly discerning between different types of drugs, etc.)

• LMFAO: 'American agents, therefore, use undercover tactics to purchase drugs; then they tip off Japanese authorities, who arrive to make arrests and confiscate drugs. Perhaps one such instance occurred in February 1990, when Japanese authorities mysteriously 'found' 24 kilograms of cocaine smuggled from Peru in a Yokohama warehouse. News reports said that the seizure was a 'matter of luck'.

Anyway, hope that helps. It probably didn't.