As instâncias do RDS são gerenciadas pela Amazon. Assim, para impedir que você divida coisas como replicação, seus usuários - mesmo o usuário raiz que você configurou ao criar a instância - não terão privilégios completos de superusuário.
When you create a DB instance, the master user system account that you create is assigned to the rds_superuser role. The rds_superuser role is a pre-defined Amazon RDS role similar to the PostgreSQL superuser role (customarily named postgres in local instances), but with some restrictions. As with the PostgreSQL superuser role, the rds_superuser role has the most privileges on your DB instance and you should not assign this role to users unless they need the most access to the DB instance.