The ABA did REQUEST that law grads not do so: http://abovethelaw.com/2011/11/any-lawyer-who-calls-himself-doctor-like-a-ph-d-should-get-punched-in-the-mouth/.

I am not defending lawyers calling themselves doctors, but unless things have changed a lot it isn't prohibited. If you have some reference to the professional code of ethics in a state that says otherwise, I would sincerely be interested in seeing it just for my own edification.