Why stop at Yahoo? Since he was a Marine I'm sure he had an official military e-mail account. Should they see those e-mails also? Most folks have more than one free account so should Yahoo, MSN, AOL etc be forced to do a search for all accounts he may have opened? Maybe he was a Freeper should they have access to that account also? They'll lose big time if this makes it to court.
If this marine had wanted his family to know the contents of his email, he would have CC:'d them. Why stop at electronic mail? Should the family find out about snail mail correspondence their son was carrying on, would they demand that the recipient forward copies to them?
Private correspondence is just that. Had their son wanted them to be in the loop, they would have been.