Why EntityTransaction?

When managing entities outside a J2EE container, you need to obtain an instance of EntityTransaction to manage transactions. Here is an example:

        EntityManager em = null;
        try {
            em = Persistence.createEntityManagerFactory("em1")
                    .createEntityManager();
            System.err.println("Created entity manager");
            Warehouse w = new Warehouse();
            w.setName("LONDON WAREHOUSE");
            w.setCity("LONDON");
           
            EntityTransaction t = em.getTransaction();
           
            boolean success = false;
            t.begin();
            try {
                em.persist(w);
                success = true;
            }
            finally {
                if (success) {
                    t.commit();
                }
                else {
                    t.rollback();
                }
            }
           
        } catch (Exception e) {
            e.printStackTrace();
        }
        finally {
            if (em != null) {
                em.close();
            }
        }

I would like to understand the rationale for introducing a new abstraction for transaction management when the UserTransaction interface already exists. By introducing a different mechanism for out-of-container applications, it will be harder to write code that is environment agnostic ... unless you write a wrapper to hide the differences.