The concept of a store is one that has been brought up many times on the forums. Its probably realized by the core dev team that creating an online store is a significant undertaking. Its quite possible that the revenue it might make does not warrant the investment of time and effort to create and manage it.
The thread on the forum about Joomla Merchandise is very interesting. It also discusses how the Joomla project has branded itself.
Drupal has their logo under a GPL, anyone can create t shirts for it.
I think that trying to control your logo within an Open Source Project is ultimately fruitless anyway. In Joomla’s case the project might as well GPL it and then allow the community to step up and run the merchandising. It seems to be working well for Drupal.