Because the wavelength determines whether you need to make an explicit effort to retrieve the information.
The search is not a matter of retrieving what is possible to retrieve, but is instead about intentionally breaching a privacy barrier.
As long as you have taken a single safeguard to protect yourself from casual observation, anyone circumventing that safeguard with the intention of observing you is performing a search.
Thus, if you want to make an FM broadcast, it would be prudent to do so on a frequency that is not expected to be routinely monitored. Anything that could be directly intercepted by commonly-available radio receivers, such as car radios, walkie-talkies, police scanners, or similar would not create a reasonable expectation of privacy. Anything that might cause interference in an electronic device is likewise non-private, as anyone who transmits should be aware that amateur and professional radio operators may investigate interference.
In those cases, sending your FM transmission from inside a Faraday cage, through a shielded cable, to the inside of another cage, would be sufficient precaution against casual observation that anyone hearing your signal must have been searching for it. I contend that even if the shielding is imperfect, the signal leaks, and the cops are able to monitor and record your transmissions, they are still peeking behind the fig leaf, so to speak. The instant that they become aware that the transmissions were intended to be private, all subsequent monitoring is a search.
Were it otherwise, a person not acting in his official capacity could breach the privacy barrier, and then later monitor under the presumption that no such barrier existed. A cop might take off his badge and shove a pin through the shielding on the cable into the center conductor, then put the badge back on and tune in the radio signal as though it were meant to be a broadcast.
The dissenting argument is ridiculous because the "searchiness" of an observation does not depend on some law of physics. It depends on whether you are exposing something that I wish to be private, regardless of the means by which you do it.