There's no such thing as legal ownership of data. There's IP rights, contracts and innumerable data regulation laws, but not actual ownership. "Ownership" here just provides a vague cover that sounds like something definite but is not.

They’re not taking the videos of people, their contractor is.

If the local police department were legally required to turn over all their computers and servers, it would only contain a fraction of what their contractor captured.

The contractor’s employees have also been proven to abuse access to the contractor’s video files across the nation. Something the local police department has little to no control over.

I believe they're implying that Flock still has access to it even if the police technically retain legal ownership.