The module doesn't clean up the it's field data in the database when it is uninstalled.
This is up to core, AFAIK this module doesn't need to do anything special about it. You will find issues about this in the Drupal core issue queue.
The message module has code in its uninstall to clean up the field instance it creates.
Oh sorry, I miss read the project name, and thought it was Message.
Automatically closed -- issue fixed for 2 weeks with no activity.
Drupal is a registered trademark of Dries Buytaert.