Interesting on many levels.
1) When have you sent an e-mail so damning you felt the need to sue an e-mail host before anyone found out about it? ala "What the hell are they hiding."
2) If I sent you a letter to your house, I couldn't sue the Postal Service to come and destroy it.
[edit]
3) If Google can just delete e-mails sent to you at a moments notice without consulting you, then it's probably a good time to start backing up your e-mail. What's to stop this case from repeating where a whistleblower sends an e-mail, gets immediately caught, and the data is deleted before read and the whistleblower is sent to jail?
4) Lastly, an elaboration of 3. You apparently don't "own" e-mail sent to you, if it's deemed accidental by a court. Which could be further expanded to you don't own anything in your inbox.
5) Side question, who owns the "sending" of an e-mail? Is it the sender, or the company he represents? If an employee sent an e-mail, would the courts consult the employee or the company? And what if the employee wants it sent?
You're missing the worst part: they E-mailed confidential information in an unencrypted message. Sending the E-mail was no mistake. Sending it to gmail.com instead of gs.com was the mistake. They are already incompetent and I'm quite sure that they leak confidential details routinely...
As for the courts complying, I believe that they overstepped their authority, but more than likely aren't technically qualified to make these kinds of decisions without consulting with experts in the field. Of course, the US government threw rights and freedoms of people to the curbside long ago. It's only going to get worse. It will never get better. Gradually, the entire country will decay and fall from its perch to the equivalent of a third world country and then they'll suddenly start trying to cooperate with the rest of the planet. Canada won't be far behind.
If Google can just delete e-mails sent to you at a moments notice without consulting you, then it's probably a good time to start backing up your e-mail.
You don't backup your email already? 
You don't backup your email already?
I'm in IT, what's a backup?
My Gmail is 16 GB. But it's shared with my Google Drive so all that stuff is backed up on their silly cloud. I've used Gmail since the beginning (when it was invite only!) and I've lost a good 3-4 hard drives since then but zero e-mails. 
3) If Google can just delete e-mails sent to you at a moments notice without consulting you, then it's probably a good time to start backing up your e-mail.
I've been doing it for years, although with interruptions. Now I'm sending myself my daily thoughts regarding stocks, including action list for the following trading day. So I've resumed the backup habbit, I've the Gmail Backup shortcut on my desktop.
Append:
My gmail is..
9.91 GB (66%) of 15 GB used
I did lose one "no lose" drive quite a while ago, didn't lose important information since then.
There's around 50GB of data I consider strictly "no loss" and there are copies in at least three places, I quit using clouds recently. Buggy & slow and/or not enough data(unless you pay quite some $).
Append1:
As of the legal stuff in the OP, I suppose they offense has proved the data belonged to the people the data is about.. And that it should be destroyed to safeguard their privacy.
Chris Katko: google data backed up on other google data?! To me that sounds like having a "backup" on the same computer.
I'd like to think that Google themselves has data stored redundantly. That said, if you have really valuable data I would be making personal backups or at least copy it to every storage device that will fit it...