Earlier quoted context omitted.
From that source: In 1986, the United States Congress updated the Omnibus Crime Control and Safe Streets Act of 1968 by enacting the Electronic Communications Privacy Act which included an updated "Wiretap Act" and also extended Fourth Amendment-like protections to electronic communications in Title II of the Electronic Communications Privacy Act, known as the Stored Communications Act . In Carpenter v. United States…
Think you're glossing over the distinction between compelling material (by warrant, subpoena, &c.) -- what the Fourth Amendment covers -- and private entities voluntarily giving the government material that third parties have entrusted them. Nothing regulates the latter: that's the third-party doctrine.
That's not true. The Stored Communications Act _does_ regulate this. In fact, it was passed in response to concerns that the third-party doctrine would mean that nothing would be protected from the government if it was stored by a third-party service provider.
The law says that the contents of communications may not be divulged unless certain conditions are met, even voluntarily. See 18 U.S. Code § 2702.