Was news to me. Turns out it's an optimization for later disabling/re-enabling. From Oracle 8i Concepts, Ch. 25 Data Integrity:
"Enabling a primary key or unique key constraint automatically creates a unique index to enforce the constraint. This index is dropped if the constraint is subsequently disabled, causing Oracle to rebuild the index every time the constraint is enabled. To avoid this behavior, create new primary key and unique key constraints initially disabled. Then create nonunique indexes or use existing nonunique indexes to enforce the constraints. "
