The data Flock holds is not owned by OP.

If I as a photographer take a photograph of someone, the photo does not belong to that person—the photographer retains the IP and ownership rights.

You have rights too, such as privacy/likeness rights, which allow you to restrict what the IP owner is allowed to do with the image that they own, but you do not own the data, and your rights give you a claim against the data owner.

Flock probably have legal obligations or contractual commitments not to delete or destroy their customers' data, and changing that is not necessarily a good thing.