I think the reason that it's a single shot instead of multi shot and full emergency braking instead of moderate is that the stops are all track based not train based, at least for the 'train in circuit' stops. The original signals basically put up a metal arm that trips the emergency brakes on the train itself so if they piggy backed off this existing system by just adding a speed sensor there's no way to do a slow deceleration because the signals are designed to work without any real cooperation from the train beyond the emergency brake functioning.