It seems to me they should just return the list of ones that are legal or at least have the option to filter the list rather than have to do it manually.
Flickr's current rule is if it's marked as copyrighted it's not to be downloaded unless it's your image. They know who you are which is how they let you add, delete images through the API.
Otherwise, they arguably need to add more permissions. They know who you've marked as friends and who you've marked as family. Facebook and G+ have no problem restricting access to images based far more complicated criteria.
How would Flickr know?