> From what I understand these systems are legal because there is no expectation of privacy in public.

Not quite. There's been precedent set that seems to imply flock and other mass surveillance drag net operations such as this do violate the forth.

Defendants trying to exclude ALPR evidence often invoke Carpenter v. U.S. (or U.S. v. Jones, but that’s questionable because the majority decision is based on the trespass interpretation of the 4th Amendment rather than the Katz test). Judges have not generally agreed with defendants that ALPR (either the license plate capture itself or the database lookup) resembles the CSLI in Carpenter or the GPS tracker in Jones. A high enough density of Flock cameras may make the Carpenter-like arguments more compelling, though.