The "explanation," at least in the city's local news, was that the shirt (pictured right) showed a Japanese maple leaf.
Nonsense. It's weed. But, it doesn't necessarily mean that Darvish may be your next smoking buddy.
In Japan, the marijuana leaf is a popular symbol without any real smoking context. Plenty of native Japanese have leafy t-shirts, or necklaces, or air fresheners in their cars, and don't really know that they're symbolizing a drug.
Marijuana is highly illegal in Japan. It's considered a hard drug, and that's why we recommend in our Japan travel guide that you leave it at home.
The Japanese justice system doesn't resemble the American one. In short, you're guilty until proven innocent and there's a 90% conviction rate in the country. A marijuana arrest would be a long, exhausting and expensive way for you to leave the country, so really, please leave it at home.
However, do feel free to buy any of the weed bling, weed posters, or Bob Marley goods you find at Japanese gift shops like Village Vanguard.