A baffling decision from a western perspective, but this must clearly be a cultural thing that I at least am not familiar with. Talking to a few people, drug charges in Japan are apparently a huge deal, and any association with individuals involved with them is viewed very negatively by people.
I'm sure we can come up with a dozen similar comparison in regards to Hollywood, such as the recent stuff with Bryan Singer and Bohemian Rhapsody.
I still fail to really understand the logic behind this, but oh well.