Those private installations are almost certainly not subject to FOIA (they wouldn't be in Illinois, hard to see how they would be in any state). Without a court order, none of them should be exposed to law enforcement, though there might be opt-out features that have that effect.
It comes down to the individual agreements that those private-contract cameras have with Flock, which unfortunately means it might be a case by case basis to understand if any one has conditions that allow sharing with law enforcement. IT was recently discovered that local police departments that had Flock contracts that limited the police department's access did not restrict general access, so Flock could still use it how they wanted and let federal agencies (ICE) use it: https://www.aclu.org/news/privacy-technology/flock-massachus...
In addition to the potential for agreements about sharing info from the server side that the sibling comment points out, there's also presumably nothing stopping the owners of the private cameras from voluntarily sharing footage with law enforcement. It's not completely implausible that the owners have them up to be able to defend against false allegations about what might have occurred in view of the cameras, but this doesn't seem nearly as likely a motivation as physical security, and barring them having some sort of private security force that they feel confident in dealing with any incidents they're unhappy with, the intent is probably to sometimes share footage with the cops.
