You are underestimating the psychological impact of culture. Good people sometimes make bad decisions because they become accustomed to what's "acceptable" around them. When you start teaching, you take on a responsibility for what you communicate, and every bit matters. No, someone's not going to suddenly decide to do drugs because of a Tyson poster. But they may decide he's a good role model as a fighter, and miss the lesson that being a good fighter isn't all anyone should be working on. And then if they miss that, they may imitate some of his behaviors and attitudes, and now they've started to learn a wrong lesson.
People are more easily influenced in small ways that is generally perceived.