What are the "Free" checkboxes for?

I've only just noticed these. The pattern of whether they are ticked appears random, and seems independent of upload status (i.e. whether the file has been approved yet or not, it can still have the box ticked). These are all time lapse uploads, by the way.

Screen image:
https://www.dropbox.com/s/2qswdvvz8wdcilh/FreeCheckboxes.jpg?dl=0

Thanks!

(by the way, how can I get the forum editor to handle links properly? It seems a bit incapable in that area!)