The difference between the legal reasoning and your reasoning is the "property in trust" part. Legally, once you hand your communication over to Google, it's their property. If they choose to hand it over to the government, that's their business.

Think of this hypothetical. Say Google reads your g-mail and tells people you ordered green shirts. Do you have a lawsuit against Google for disclosing your private information? I doubt it. If the information is sensitive for a different reason (e.g. it's about your herpes diagnosis), you might have a suit, but then you'd also have a suit against a friend to whom you told that information in confidence. I.e. it's a suit based on the nature of the sensitive information, not the disclosure by itself.

The idea of this "information trustee" doesn't really exist in the law. Google is treated no differently than a guy on the street you tell to relay a message to your friend. And as for the 4th amendment, as I said, wiretapping aside it's more of a "personal search" protection than a "communication protection." This is a good summary of the relevant precedent: http://www.cs.duke.edu/~chase/cps49s/carnivore-history.html

  | Legally, once you hand your communication over
  | to Google, it's their property.

If I put some of my belongings in a storage unit, I don't lose ownership of them. If I accidentally leave (e.g.) my sunglasses at a restaurant, the restaurant doesn't own them now. If I give a package to DHL, they can't decide that they don't want to deliver it, and it's theirs now.

There is in the law the idea of trusteeship of personal property (the law of bailments). There is no such analogue for information.

I'm arguing that the analogue should be there. When I mail a letter to someone it's just a physical manifestation of the information. The fact that it's physical is a by-product of the transfer medium. I don't feel like rights should be lost just because we can represent the content of that letter as an email, and send it over a 'series of tubes.'

If I store files on Dropbox, I am arguably paying for them to store the files for me, not giving them to Dropbox to do with what they like. It should not be treated any differently than a rented storage space in the physical world (regardless of implementation details like de-duplication).

Here is where you get into the tricky difference between what makes sense, and legal definitions. You may very well be right- I certainly feel you are. But that line of reasoning must be established by legal means.

It's like right now you have the idea, but haven't written the code.

But if you leave your sunglasses in a restaurant, and the government wants to take fingerprints off of them, they can.

The statement that once (e.g.) Google has the message that they are supposed to be passing on my behalf, that it's now theirs to do with as they please is a much more bold statement then the original discussion of whether or not they should be allowed to hand it over to the government.

Not really. That is nothing more than a civil contract issue between two parties. And to have a valid contract there must be an offer, acceptance, and consideration. Not just the promise of future consideration either, so saying that Google could make money off of you later (by e.g. directed advertising) wouldn't make an enforceable contract.

But as it stands things are even more lopsided towards Google. Go check their terms of service, I'd bet they have a half dozen different ways of indemnifying themselves from liability in the event they don't deliver that email.