The problem as I see it is this.
The internet allows us to put forward much information on a unregulated medium, which (not specifically) todays youth seem to think its 'okay' to get a million peoples feedback, instead of simply getting the important peoples feedback. How many times do you see facebook/twitter/tumblr/google+/myspace/(insert social medium here) posts which you know the individual would most likely NEVER, EVER speak to the person the message was intended for. If you seriously need to vent something and don't want anyone to find it, write it on a piece of paper, get your frustrations out, then immediately shred it.
I wish that all the folks who think social media is the place to vent and write obscene comments like the letter she posted could learn the valuable lesson that she did. ANYTHING posted on a social medium (especially the internet) can be found by ANYONE accidentally or otherwise, and these documents are always, always backed up, and shared with other sites, etc. I will bet you anything she will think twice before she posts anything from this point forward.
For those who think this is abuse, that's your opinion. I for one believe that the gun was overboard, but if he destroyed it any other way, he would still get flack for it. As a teenager, she is trying to find her mark in the world, secretly abusing (verbally or otherwise) the people who support you and put a roof over your head is not the way to make your mark in the world. It is, however a great way to piss those people off and give them a reason to not give you the luxuries that life has to offer. Mind you, luxuries are anything that doesn't include shelter, food, education, or clothing. Even at that point, many many children in this world would like to have those great luxuries. 'Beg' for luxuries? sure, why not? I'd bet this guy makes about 25-40 / hour. I bet he pays over 75% of that to maintain their home, car, health insurance, put food on the table, clothes on their backs, etc. When you break that down, you really get about 5-10/ hour of 'disposable' income. Yes. I would make my children ask for luxury items. If I have to work 60 hours to get them a laptop, I will make sure they will take care of it, they will not abuse it, etc. Even $20 for a movie is 2 hours of my work time to give my child 2 hours of leisure, not a big deal, if they can prove to me that they are doing their part to ensure that the family's livelihood is being maintained. It's a respect thing. They respect my wishes to have a happy, clean home, and I respect their wishes to have some leisurely fun. It's as simple as that.
I am sickened by how many people think that she 'needs' a car because they live in a 'rural' area, or how she 'needs' a laptop to survive... A 15 year old does not need these things, they want these things. When it comes to things you want, you earn them, you do not abuse them, and you use them properly for the purpose they were intended.
I highly doubt the laptop was purchased with the intent to use facebook on it, much less to publicly slander her parents to all (400+?) friends on her facebook(minus family). In reading more about this story, he originally posted this story on her facebook page, which ended up going out to about 40 more people. Those people blew it up... Unfortunately, we also don't know that her tirade wasn't shared with many more folks as well, because as a society, we just brush off a teenagers blatant disregard for her parents as a teen being a teen. B******T. Being a teen isn't an excuse for being dumb.
As for putting it online, I half heartedly disagree with it, but only because he had already tried other means of fixing the problem. When that failed, he went to teach her a lesson in humility. He decided that reading the letter, and responding to her complaints, then showing her her punishment on the medium which she seemed to enjoy was the proper punishment, and I can't blame him. In my opinion, if it worked, good for him. She was not hurt in the process, and now she will either be smarter about her 'privacy' settings, or (hopefully) she will learn what constitutes a real issue and what is frustration and how to deal with it appropriately