This was a choice made by the page's author, simply by omitting a button or graphic that creates a new item. Second, the black triangles are nowhere in sight. Lightweight editing allows people to contribute to a page as easily as possible. Editing directly in the web browser. Editing one item at a time.
Contributors add or edit just one item at a time.
During editing, the context of the rest of the page remains in place. Meanwhile, the page contents above and below the editing region remains unchanged and visible. The Sparrow page author specifies the fields in the Sparrow items, and Sparrow presents a form for contributors to fill out or edit. Contributors simply fill out forms rather than see the actual HTML that is generated to format their inputs. The author has already specified the formatting for the Sparrow item.