NWS on the Airbus FBW is steer-by-wire, that is, there is no mechanical connection between the tiller and the nose gear, but it has sensors and feedbacks to prevent the nose-wheel angle going too far, which prevents damage, even if the tiller is moved further.
The tug has no electronic limitations itself, and the driver could easily force the nose-wheel too far round. I cannot remember if ECAM brings up a steering angle caution when under tow - never having seen it in practice.
That or the limit lines painted near the nose-wheel are to stop the tug driver going too far each side, and the lower angle limit presumably provides a safety margin, allowing a slight overshoot while the driver realises and corrects.