1. Why would DDL statements not roll back?
2. Why would DDL statements not be written to roll back segments?
3. Why can DDL statement roll back?
DDL statements used to not be written to rollback segments. But many DDL statements modify the Data Dictionary. And the modifications of the Data Dictionary are written to the roll back segments (or Undo tablespace). These modifications are also written to the online redo logs. If I recall correctly, more and more DDL statements are being written to the online redo logs to aid in recovery efforts.