Because even if they are doing something immoral, I'd be an idiot to start criticizing them for it if I wasn't perfect myself. Smoking is self-destructive. Drinking is self-destructive. Losing your temper and yelling at people is wrong. Lying is wrong. Cheating is wrong. Stealing is wrong. But people do that stuff all the time. Soon as I figure out how to be a perfect human being, then I'm qualified to go lecture other people about how they live their lives.