If you add a roster to a course worksite, then remove it, the roster will be marked as still in use the next time someone comes to the roster selection screen in worksite setup. This is somewhat mitigated by the fact that one can "override" this by clicking "Use this roster anyway?" link, or one can delete the site altogether, but it can be confusing.

I have confirmed with a tiny bit of database analysis. I observed that after disassociating a roster with a course site, there were still many rows in the various sakai_realm_* tables associated with the course rosters. I tried doing one thing which was going to the SAKAI_REALM_PROVIDER table and removing the rows in this table associated with the rosters I had removed. After doing this, the rosters no longer appear to be used when creating a new course site.

I have also confirmed this bug does not exist in the 2.6.x or trunk (future 2.7.x) branches.