You may not be aware of it, but Flickr actually has something called the Wall Art Store, where you can purchase prints of photos in various physical formats. For a while, Flickr was including some Creative Commons images in this store, which meant they were making money off of photos they weren’t paying to use.
While it seems they weren’t technically doing anything outside the rules, many photographers saw the whole thing as a bit sketchy. As the Flickr blog post puts it, “…including Creative Commons-licensed work in this service wasn’t within the spirit of the Commons and our sharing community.”
The marketplace actually works on a system of other photographers who opt in to having their work sold in the store. Then, they’re given some of the profits from the sales, which is how it should be. Because the Creative Commons images were available as free to use, their creators weren’t getting that cut.
While Flickr did take some flak for the whole thing, they seem to be intent on making up to the community.