The key to the circumcision example in Box 7.3 and Figure 7.9 is the direction of causality. Usually we have Disease causing Test result; in this case we have risk factor causing disease.
Since we've mostly been dealing with test results rather than disease causes, we've been OK assuming LR don't change much (except in all of the situations we've described) with prevalence.
This is why LR are not got for risk factors and OR are not good for test results.
I've updated Box 7.3 to clarify these points and I uploaded it right under Chapter 7. Let us know if you are still confused.
Tom