Do the checkmarks do anything? I expected them to disappear after a reload (like hiding a post on Hacker News), but apparently that's not what they're for.
"Mark as read" checks all the checkmarks, but since they're still there after a reload, I don't see the point.
The really weird thing to me is that the check marks don't persist across categories. I.e. I marked the story about Youtube as "read" in the "World" tab, but it doesn't get a check in the "USA" tab. Seems like the feature is pretty half baked.
It automatically marks stuff you've seen - it's just a visual cue. Similar to how search engines (like Google) show visited links with a different color.
I think keeping them on the page instead of automatically hiding them makes more sense for a product that's trying to update their news feed once per day. You feel more in control, as if it's not a stream of never-ending stories, but rather a fixed amount of stories that you can realistically power through. Seeing all items checked sort-of supports this philosophy.
"Mark as read" checks all the checkmarks, but since they're still there after a reload, I don't see the point.