If high profile people cannot be bothered to change the PIN to their voicemail they shouldn't be terribly surprised that hacks dial in to their voicemail. Hacks are known to be pretty good at getting to info they want. If you leave default security level in place that a five year old could work out, a journo isn't going to have too much of a struggle to listen to your messages.
Sure, it's not legal, but if there was a general default email password on accounts that nobody bothered to change and hacks logged into that, we might be asking questions of the "victim", and why they made it so easy, and not just why hacks are so devious.
And, after the first round of phone security was published, surely no high profile person keeps their voicemail and the default setting now?