Why is it disadvantageous in graduate admissions to have a higher degree?
Berkeley offers the following rationale for their policy:


  The Graduate Council views academic degrees as evidence of broad research training, not as vocational training certificates; therefore, applicants who already have academic graduate degrees should be able to take up new subject matter on a serious level without undertaking a graduate program, unless the fields are completely dissimilar.


Although this is rather vague, it does suggest that they believe that when one already has a PhD, it is pointless to enter another graduate program in a similar field, as one has already demonstrated "broad research training" and shouldn't need more.  And since PhD program spaces are a limited resource, and involve a considerable investment of faculty time and university resources, they don't want to spend it in ways that they consider pointless.

That's my interpretation, anyway.