The very first holding of the majority opinion by Kagan:

Held: Police officers conducted a Fourth Amendment search when they acquired Chatrie’s location data from Google because an individual has a reasonable expectation of privacy in his cell-phone location information.

Note the possessive “his”. Crucial to the case, this was held to be the individual’s data, not the third-party’s.

Why would the expectation of privacy be different depending on which spectrum of light the information was captured in (visible vs radio)?

In both scenarios, the data is held by a private third party and a person generates this data pretty much by-default.

This is the relevant bit:

In Carpenter, this Court held that accessing cell-site location information (CSLI) constitutes a Fourth Amendment search because “individuals have a reasonable expectation of privacy in the whole of their physical movements,” 585 U. S., at 310. The Court reasoned that CSLI provides a “detailed” and “encyclopedic” portrait of a person’s whereabouts, id., at 309, and, with that, “an intimate window into a person’s life,” id., at 311. Because people “compulsively carry” their cell phones “all the time,” the Court explained, a cell phone “tracks nearly exactly the movements of its owner,” and thus “faithfully follows” him not only through “public thoroughfares [but] into private residences, doctor’s offices, political headquarters, and other potentially revealing locales.”

I'm not sure that's quite as clear as you say it is. It could be "his" as in "about the person" rather than "belonging to the person".