It's not so much whether you make money or not--it's whether they lose money. In the case of someone painting Disney characters on the wall at a school, Disney could have made a wallpaper mural that they could have purchased, then Disney would have made money. However, the school case is a tricky one, because Disney doesn't want their characters displayed as if they're endorsing that school. That's another way Disney could theoretically make money with schools. There's no reason that they couldn't make their own line of Disney-accredited preschools, so the one that got sued couldn't use their characters.
Companies do have to be vigilant about defending their copyrights. If they set a precedence of just not caring, when they do decide to sue someone, that person can cite examples of when the company didn't sue, and that person can win. I think that asprin is one product that used to be a trademarked company name, but they didn't defend it, and now it's just a common name.