Whenever a Pinterest user pins something to the site, they are prompted to "describe your pin." This small detail that prompts a caption to be added to all photos might be what keeps Pinterest from getting hit with mass copyright infringement suits, according to Itai Maytal, an associate attorney who specializes in media and intellectual property, at New York law firm Miller Korzenik Sommers.
If a user tries to pin something without adding a caption, they will get an error. The point of the caption is to basically force the user to add a comment on the photo, which allows Pinterest to make the argument that any pictures on their site are protected under fair use, a limitation to the exclusive right of the owner of an image. Maytal states that "it would favor them (Pinterest) because it's commentary, and commentary and parody are some of the types of uses that the law encourages. It certainly wouldn't end the analysis, so the jury would still be out as to whether this is all protected under fair use."
Pinterest has been going through some scrutiny regarding its pinning process as of late, where users can add full versions of an image to the site, without a license. Flikr recently began adding no-pin meta tags to certain of its images to render them "unpinnable."
Pinterest (“Pinterest”) respects the intellectual property rights of others and expects its users to do the same. It is Pinterest’s policy, in appropriate circumstances and at its discretion, to disable and/or terminate the accounts of users who repeatedly infringe or are repeatedly charged with infringing the copyrights or other intellectual property rights of others.
You acknowledge and agree that you are solely responsible for all Member Content that you make available through the Site, Application and Services.
Basically, Pinterest hold its members solely responsible for what they pin and repin, and backs this up by collecting user comments before anything can be added. Pinterest still claims to be attempting to sort out its copyright questions, which may come to prevent the site from moving forward at some point in the future.