You are right that this is new and a change. An IllegalArgumentException should cause tx to be marked for rollback only. If you look at the JPA 2.1 specification section 3.1.1 EntityManager interface description, you will see this small paragraph (bottom of page 79):

Runtime exceptions thrown by the methods of the EntityManager interface other than the LockTimeoutException will cause the current transaction to be marked for rollback if the persistence context is joined to that transaction.