How did a device that originally printed Morse Code dots and dashes onto paper tape come to represent all devices that essentially provide a weak wiretap? Cast your mind back to a time before the Internet and before the phone network. Before both of these, the advanced communications technology of the time was the electronic telegraph (The Victorian Internet)
Pen registers (also called ‘inkers’) passed the incoming telegraph signal through electromagnets which moved an ink arm that pressed the paper tape against an ink roller. This produced a role of paper with the incoming dots and dashes of Morse Code. Pen Registers were later used when the phone network was created to record sequences of phone dialing pulses. Pulse dialing worked by interrupting a direct current in a standardized way. (Modern landline phones use multi-frequency tones (tone dialing) to send digits across the network). Eventually, we came to our modern understanding of pen registers as any device which records or decodes electronic or other impulses which decodes dialing, routing, addressing, or signaling information for a communications technology.
Surveillance using pen registers and/or trap and trace devices is basically a weak wiretap. It allows the collection of lots of information about communications without collecting the contents of the communications (See the EFF Surveillance Self-Defense site for more information). When used with the telegraph system the pen register essentially recorded the contents of the communication. When used with the phone network (as the Pen Register Act intended) the pen register only captures peripheral information about who is called and when and how long. Now with the Internet a pen register captures ports, protocol, and header information. While this falls short of capturing the contents of the communication, it certainly reveals much more about the communication than is discovered when monitoring the phone network.
The Pen Register Act is part of the Electronic Communications Privacy Act of 1986. The act was partly a reaction to the Smith v. Maryland case (442 U.S. 735 (1979)) in 1979 where the Supreme Court reasoned that because you knowingly expose phone numbers to the phone company when you use the phone network, the 4th amendment will not protect phone numbers against surveillance by the government.
The Pen Register Act reestablished a reasonable expectation of privacy by requiring the government apply for permission with a showing "that the information likely to be obtained is relevant to an ongoing criminal investigation being conducted by that agency." (3122 Application for an Order For a Pen Register or a Trap and Trace Device) This is a much lower standard than probable cause and no facts are needed as part of this statement, just an assertion that it is likely to be relevant. Still, it does provide for some reasonable protection.
Our understanding of past technologies inform our policy for regulating modern technology. Perhaps these past technologies aren't a good model for understanding our current technologies but they will be looked to as a guidepost for crafting modern privacy expectations.