I made one of these back in the day. Also allowing positive selections gives the model a lot more information and gives the user more power. You can use that to easily remove gridlines and smudge marks in photos of drawings without having to select every little thing you want deleted, or similarly you can positively select the main subject and a bit of scenery, negatively select a few passers-by, and let the model remove a crowd. The key point is that it moves some of the training-time bias toward a runtime selection, allowing better results on a wider variety of tasks (at the cost of more clicks on photos the model understands with few removals happening).