Your assumption is right. The finasteride helps the non-transplanted hair that is still present on top, but is susceptible to male pattern baldness. It indirectly helps the transplant, simply because everything looks a lot fuller several months down the road if the native hairs aren't dropping out around the newly transplanted ones. The surgeon ends up getting credit for both, even though the medication is responsible for one of the two positive effects. On the other hand, we often get blamed when a transplant fails to meet the patients expectation for additional density, sometimes not because the transplanted hair didn't grown in great, but rather because of the gradual thinning of the ones that were there to start with.
Mike Beehner, M.D.