>In theory, the data was provided willingly when collected.

That's spot on, and your analogy is a good one, except that in the realm of personal information, no warrant is required in the US.

There is quite a bit of law and numerous court decisions around this process in the US.

That jurisprudence is more generally called the Third-Party Doctrine[0]:

   The third-party doctrine is a United States legal doctrine that holds that 
   people who voluntarily give information to third parties—such as banks, phone 
   companies, internet service providers (ISPs), and e-mail servers—have "no 
   reasonable expectation of privacy" in that information. A lack of privacy 
   protection allows the United States government to obtain information from 
   third parties without a legal warrant and without otherwise complying with 
   the Fourth Amendment prohibition against search and seizure without probable 
   cause and a judicial search warrant.[1]

[0] https://en.wikipedia.org/wiki/Third-party_doctrine

Edit: To clarify, I disagree with this doctrine and would love to see limitations on data retention periods as well as warrant requirements for access to such data.