This hat was given out to people who bought the Rare PSP Launch Day Shirt as an extra bonus. This DLC is not available anymore.
The reason for this is that the Rare PSP Launch Day Shirt was made free in certain regions by accident. As compensation, anyone who bought the Launch Day Shirt originally (without receiving it for free) was emailed so that they could receive the cap.
This means that the cap is able to prove that the player originally purchased the content, instead of getting it due to a mistake.