This mug is also available to see in the High End Sonic Collection (TODO:LINK).
Why? Because it is very rare. It was only available for sale in Segaworld Sydney, and was probably a bit pricey at the time. Very few of them exist today, so even if you want to collect one, you'll have a lot of difficulty. It's nice to finally be able to get a look at this mysterious mug. You can read more about it on the High End page.