> It’s amusing that the Government calls what they’re doing with Google a “pen register”. They usually do that to squeeze through the small exception in the Fourth Amendment created by Smith v. Maryland.

Well, they do it most directly because it is a “pen register” as defined in law: …the term “pen register” means a device or process which records or decodes dialing, routing, addressing, or signaling information transmitted by an instrument or facility from which a wire or electronic communication is transmitted, provided, however, that such information shall not include the contents of any communication, but such term does not include any device or process used by a provider or customer of a wire or electronic communication service for billing, or recording as an incident to billing, for communications services provided by such provider or any device or process used by a provider or customer of a wire communication service for cost accounting or other like purposes in the ordinary course of its business;… 18 U.S.C 3127.

You are correct that this statute was created to manage the process related to the exception identified in Smith v. Maryland, but I don’t think that there is a good-faith argument that it is either inconsistent with the scope of the exception or anything except a limitation on law enforcement compared to not having the statute.