Politicians live in the public eye, in a way different from celebrities (be they pop stars or CEOs). They are, in a sense, obligated to grant interviews and be open to the hard questions, because a politician is selling him or her SELF.

A pop artists sells music, a company sells a product or a service. I'm not predicating my use of facebook based on whether or not Mark Zuckerberg did pot in college or got blowjobs from his secretaries.

So when you interview someone who isn't selling themselves, you don't have the freedom to ask those kinds of questions. Unless maybe you don't that person to ever grant you or your magazine an interview ever again.