This would probably constitute illegal wiretapping and would certainly constitute a 4th Amendment search if conducted by law enforcement.
Normally, the definition of whether something is "public" for purposes of an expectation of privacy is whether it could be detected by a human being unaided by technological enhancements from a place where someone could lawfully be to make that kind of observation.
Some of the relevant cases are Katz v. U.S., 389 U.S. (1967) (tape recorder outside a public telephone booth was a search violating the expectation of privacy) and U.S. v. Karo, 468 U.S. (1984) (tracking device placed in barrel by authorities violated expectation of privacy).
RFID signals are not "public" even if they are not encrypted with a private code because a device, such as the ones identified in the question, is necessary to receive them.
The Wiretap Act, codified by 18 U.S. Code § 2511, is a federal law
aimed at protecting privacy in communications with other persons.
Typically, when you think of a "wiretap," the first thing that comes
to mind is someone listening to your telephone calls. But the Act
protects more than that. Under the Act, it is illegal to:
intentionally or purposefully
intercept, disclose, or use the contents of any wire, oral, or electronic communication
through the use of a "device."
The Act provides criminal and civil penalties for violations, although
it creates various exceptions to when interceptions and disclosures
In this circumstance, despite being passive, one is intentionally intercepting the contents of electronic communications through the use of a device. The fact that there was not in all cases an intent to communicate through, for example, an RFID chip, on a specific occasion probably does not suffice to render it not a communication.