IMSI catchers aren't observing things in plain sight; they're invasive searches that the Fourth Amendment prohibits outside of narrowly-defined circumstances.

The First Amendment precludes protected political speech from being used as a basis for such a search.

The Fourth Amendment further prohibits dragnet searches of indefinite groups of people, such as a protest, because it requires a warrant to "particularly describe" the "persons or things to be seized". (The "Particularity Clause").

I fully agree with your comment in the different case, which is not this case, where government merely passively observes things happening in a public space. IMSI catchers are different; one way being, in that a Stingray *actively interacts with* a device, without authorization, by sending it corrupted packets. (So I understand). A second way being that it violates general social expectations of what's in "public" and what's in "private"; by analogy, if police used laser microphones to listen in on faraway conversations; or in public crowds, used terahertz radiation to look under people's clothes; those are non-public searches, any pedantic interpretations of physics notwithstanding.