To understand why it is not mandatory to wear a helmet on a motorcycle with a self-protection structure, it is important to consider the safety features of these vehicles.

- **Motorcycles with self-protection structure**: These motorcycles are designed to offer additional protection to the driver. One of the key features is the inclusion of a **seat belt**.

- **Seat belt**: In two or three-wheeled vehicles that have a seat belt, the use of this device is mandatory. However, when the vehicle is equipped with a seat belt, **it is not necessary to wear a helmet**. This is because the seat belt provides adequate protection in the event of an accident.

- **ITV documentation**: It is crucial that the motorcycle is properly indicated in the Vehicle Technical Inspection (ITV) documentation as equipped with a seat belt. This ensures that the vehicle complies with the necessary safety regulations to dispense with the helmet.

In summary, if a motorcycle with a self-protection structure has a seat belt and is correctly documented in the ITV, it is not mandatory to wear a helmet.