Sensorvault

Likely it exists for reasons of law enforcement and/or corporate data hunger, framed as policies promoting aspects of "safety" and/or "efficiency of the market".

It makes less sense in the day of computers and the internet, but I think the reasoning seems to be that law enforcement asking someone that you voluntarily gave information to, for that information, is not a search of you or your property in the 4th amendment sense. So if the that someone gives that info to law enforcement, there's nothing you can really do to claim they got that info by unreasonably violating your privacy.

Or, to put it another way, the government needs a warrant to prove searching through your stuff is justified, but you cannot compel others to keep information private on your behalf, or have it count as your information instead of theirs, if they do disclose it.

This made more sense before computers because, by and large, the primary way for third parties to get information that would be considered private would be through voluntary, and considered, action on your part. e.g. your bank holds your banking records to facilitate your banking actions, or you tell the phone company a phone number so that they can connect you. Law enforcement can then ask the bank for your bank records, and the phone company for the numbers you've dialed. It's not searching you per se, but is rather what the third party has learned from interacting with you.

Today however, thanks to the internet, computers, and cellphones, we practically hemorrhage information into the wider world more or less non-stop with only the barest awareness we are doing so.

The legal precedent come from a time before digital records and generally available encryption so it was largely true that you didn't have a general expectation that papers in the hands of third parties would be private to you, inherently they possessors would have knowledge of what was on those documents. Beyond that it just hasn't really been revisited in depth since the advent of generally available encryption and the internet. Some states have actually passed laws extending the 4th amendment warrant protections to information held by 3rd parties though.

It’s stored communications data held by a third party to the communication. In the case of email, you have sender, receiver, and Gmail. In the case of SMS, you have sender, receiver, and Verizon.