OK, but I have two issues with that: first, for every Jenny McCarthy there are multiple thousands of ordinary parents. For crying out loud, she's not exactly an exemplar of anything!
Second, it is the medical community's responsibility to be transparent and open and if that means dealing with cranky scared parents, that's part of the job - especially since at least some proportion of cranky scared parents come by their fear and crankiness from bad experiences. Dentists, in particular, shouldn't get into the profession without knowing they have to deal with phobias - fear of the dentist is a common one. Doesn't really matter if the phobia is the parents' or the kids' - dentists should be prepared to help them through it.
I can see legitimate explanations for this policy, for instance, that a nervous parent might make a child nervous - but then they owe the parent a clear explanation of exactly why their policy is different from other dentists, and they should probably acknowledge that they are not the best fit for certain situations (e.g. a special-needs child who may need an advocate to navigate an unfamiliar situation.)
Ideally, if the parent can't be in the room, they should offer some alternate means for transparency, like a one-way window, (most therapists we've used have them.) or even a webcam.