This is also a proper diagram. Notice that it differs from the two variable regression diagram shown in the book (and above) in that there is no sling joining the two predictor variables. This diagram demonstrates a property of structural equation models. They have many strengths in that they allow you to incorporate additional information into you models (in this case, we are assuming that it is known that the covariance between A and B is zero). This may happen, for example, in cases where the predictor variables are manipulated by the researcher, when the researcher has systematically sampled individuals so as to produce a zero correlation in the sample, or when it is well-known in the area that these two variables are unrelated. On the other hand, to the extent that this diagram makes an unwarranted assumption about the data, it may lead to faulty conclusions.