I find it rather disturbing when a news agency publishes information that is personal, and illegally obtained. Just because the person involced is a public person, they have no right to print a story based on stolen material.
While it is perfectly understandable that people who hold a public interest are obviously more newsworthy; they should still have an expected level of privacy. This act no different than a camera crew planting a camera in Prince Charles' bedroom and then broadcasting it over the 6 o'clock news. I understand that it wasn't the The Mail that stole the diary, they did publish the diary, knowing it had been stolen.
I can see no other ethical reason to publish the contents of the diary. I see no loyalties to anybody but the writer himself and the company that hired him.