A free press is a valuable thing, but relies on responsible self monitoring and behaviour by the media industry. Being given private correspondence, the publication of which is not only not in the public interest but damaging to relationships with other nations, allies even, is a situation where one ought not say, "Oh good, a scoop, that'll sell more papers for a bit regardless of consequences, freedom of the press allows me to publish anything and be damned", but realise that it needs being returned to where it belongs. If an individual chooses to be irresponsible then it is fair enough that they and the incident are investigated to see if the law has been broken.
In this case I see no positive side to the published information that could justify making it public. It has caused many to find themselves in "impossible" situations trying, at the same time, to both support the guy correctly doing their job, and to repair/limit bad feelings between nations.
In any case the publisher concerned has been seen to be unfit to make the decisions they need to in their job; and if the media industry supports them, the industry will have indicated that they are unable to self regulate; and maybe Westminster needs to reconsider if the laws that frame press freedom need to be reviewed.