This article explains that owners or moderators can ban members for any reason, and directs members to contact the owner with their concerns. It can also explain why you may have been banned.

If your member ID or email address has been banned from one of the groups that you are a member of or are trying to become a member of, you will need to contact the owner of the group. Please keep in mind that group owners have total control over their group membership and have the ultimate say on who they allow (or do not allow) in their group. They may ban or unsubscribe a member's address from their group for any reason they see fit, even if the group is listed as a public group.

The Yahoo! Groups Support Team does not have any say on who the moderator allows in his/her group.

To contact the group owner, send a message to:

GROUPNAME-owner@yahoogroups.com

Note: Please replace "GROUPNAME" with your group's name.