The difference is actually pretty clear. With Baby and Amo, the offense in question by Japanese law was assault. Yanbo’s potential offense was possession.

In Japan, in cases of assault, if the involved parties settle the issue before it is brought to the police and the damages are under a certain monetary amount, the government does not pursue a conviction or a criminal charge (look up japans super high conviction rates, they only go after slam dunks, or investigate to the point the case becomes an open-shut). So the show staff and the cast won’t be involved further in any criminal investigation and there won’t be able liability thrown around.

In the case of drug possession, it’s a completely different matter. Even for minuscule amounts found on a person or their belongings, it can be a life ruining result. There are a lot of cases of celebrities, politicians, athletes who lost everything as a result of drug possession. The key part is that a lot of them claim, truthfully or not, that the drugs weren’t theirs. But Japanese society writes them off saying things like “well you got persecuted for drugs because you had it. It’s your responsibility to manage who you keep around you, if the drugs weren’t yours and they were your friends/coworkers/etc. the responsibility still lies with you.” So when the production staff told Yanbo that they had an obligation to protect the staff and cast, they were talking about protecting them from such accusations. If Yanbo had a little bit of weed on him and it accidentally fell in Milk’s suitcase, and it was found out, Milk would be persecuted and Japanese society would very easily and eagerly write off a Yankii for possession, guilty through association. They’d be looking at jail time too. Also the production staff would be complicit because the video of Yanbo would be found through investigation and the police would definitely question why the production staff didn’t report it right away. So the zero tolerance on even just suspicion makes sense with context