That's another fine line - between a celebrity who thinks their fame makes them into an expert on every subject and one who decides that if they're going to have all this influence, they might as well use it to promote what they consider good causes.

Example of case 1, the genuine expert on his own subject who's turned into an ignorant blowhard on topics he knows nothing about: Richard Dawkins. Example of case 2, the celebrity who uses his fame to promote good causes: Sean Penn.

I haven't been following Rowling in detail, but I think she falls into case 2. Even if you disagree with her on Scottish independence.