Well if one wanted to interject several what ifs in this such as the parents getting a court order to retain the data; the person in question was also a marine who died in a battle zone. Depending on the intended recipient (a fellow military buddy?), some of the emails might contain information in which not only did the soldier not intend for it to be released, but the military wouldn't want to become public "in the interests of national security". If it ever came down to an argument like that, one could be assured the military wouldn't sit down without a fight. Then again, the article also says that all letters which would have been recieved are sent along, and all letters which were intended for sending would be delivered to their target. No mention was given to making sure no sensitive information is contained in there, though I at least would expect the military would have something in place in cases where a current operation contains secret info they'd rather not get out...
In either case, I wouldn't want my parents to know everything about myself. I'm just not that public of a person and tbh am a tad reserved. If I want someone to know something about myself, I'll tell them. If I were missing but showed up at a latter time (OK this guy is deceased, but bare with me), I'd probably be a tad bit annoyed. Annoyed enough to return it to the courts and argue against the former rulling? Not sure, though by that point, the damage would already have been done.
In the case of someone over 18, I'd definitely say no, it's confidential.
Under the age of 18, I'd largely say the same thing except with certain provisions. AKA, if the child is missing and the contents of a given email could be of assistance in finding them...then in that selected case with that selected email, OK. A judgement call would have to be made, but things that look to be secrets shared among friends that have nothing to do with locating them, I wouldn't just release.
Another such case could be if information there might be needed to save their life...
Course, in these cases, someone would have to review it to determine what should be released and what shouldn't... That might or might not be an issue for some kids.
There are other ways one can keep things confidential. In another forum I remember reading of one teenager (I guess) who had problems with extremly nosey parents. People suggested they keep their diary or what not in Kanji. (OK this was an anime forum...and it isn't beyond reason some peeps might not learn Japanese there, as I'm actually doing to watch some of the original shows without needing the sub-titles.)
Thing is, for kids it can be relatively easy to learn a new language. In childhood, the areas of the brain responsible for language skills are still forming... For adults, one can learn a new language, but it can also be a bit more difficult then in childhood. If an adult's only language is English, learning something like Kanji (which would be like hierophyphics where each kanji represents an idea and not a grouping of letters would probably taking some adjustment). I've been told by higher level Japanese students that one would need to learn about 60,000 kanji to be fluent.
In my case, I ended up needing to take Japanese 101 over again (at 32 now, it was a bit difficult at first), though this second time I did quite well and have hit a point where the difficulty seems to be gone and it just suddenly became easier about a month ago or so... I guess I just needed to get a certain foundation down, then the rest could just come to me. At first, it wasn't quite the same as being exposed as a kid and just naturally picking it up however... I haven't learned kanji yet, but I imagine by the time a parent would learn this, the kid would be much older...