Looks like according to USC 18, internet / phone communications affect interstate or foreign commerce and are therefore are "electronic communications". That makes listening to them fall under the definition of using a "pen register", which is prohibited.
However, walkie-talkies do not affect interstate or foreign commerce. In that case, is it legal to record their meta data?
18 U.S. Code § 3127 (1) the terms “wire communication”, “electronic communication”, “electronic communication service”, and “contents” have the meanings set forth for such terms in section 2510 of this title;
18 U.S. Code § 3127 (3) 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. Code § 2510 (12) “electronic communication” means any transfer of signs, signals, writing, images, sounds, data, or intelligence of any nature transmitted in whole or in part by a wire, radio, electromagnetic, photoelectronic or photooptical system that affects interstate or foreign commerce, but does not include—