I think people nowadays believe that those in the lime light have to be role models, because so many are faceless empty shells with no personality they need someone to look up to and be like.
so even if most people have smoked the odd joint, those in the celebrity spot light have to conform to a certain set of rules that the public doesn't. Sure a lot of people are down to earth and don't give two shits, but a lot of people will be crushed, "My precious Nigella has smoked WEED? What will I do now?"
Like the Jimmy Carr tax dodging thing last year. It's only national news because he's a celebrity. Many people probably have done it and still do it, it's just that he got caught, and he's the one that took the flak from the public and the media.
the media needs people to scrutinise so everyday people (the empty hollow shells I mentioned earlier) can blame someone else for how shit their lives are. They don't want to take responsibility of their own problems so the media makes everyone happy by pointing the finger and going "WANKER" so we can all follow suit can call him a wanker as well.