You're missing a big point though. Does Pintrest need to internalize and take ownership over all of the content on the site in order to protect themselves legally? It seems that would open them up to far greater liability. And then in the same breath they foist that liability onto their users simply for doing what they are 'supposed' to do (pin things they are interested in).
As others have said, Flickr does not have the same "we own it now" stipulation so don't suggest that this is simply the way it has to be. If Pintrest were so inclined, they could keep the indemnity boilerplate but stop taking ownership of posted content. This would move their site much closer to Fair Use territory, which would in turn help minimize the legal risk taken on by users who pin and comment on content that isn't their own. Instead, Pintrest want to make sure they don't close a potentially lucrative door and don't mind if they abuse everyone involved in making and sharing content while they explore their options.
As others have said, Flickr does not have the same "we own it now" stipulation so don't suggest that this is simply the way it has to be. If Pintrest were so inclined, they could keep the indemnity boilerplate but stop taking ownership of posted content. This would move their site much closer to Fair Use territory, which would in turn help minimize the legal risk taken on by users who pin and comment on content that isn't their own. Instead, Pintrest want to make sure they don't close a potentially lucrative door and don't mind if they abuse everyone involved in making and sharing content while they explore their options.