> Nothing regulates the latter
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.
