Not allowing residents to bill directly is exploitive of residents. Group private practices hiring  mostly residents and interns are, most of the time, a sign of someone/s making a decision to exploit people (residents) and make profit. Many practices offering this model are known to offer sub-par supervision. In fact, I believe the required training to become a supervisor is what needs to much more rigorous. Limiting how residents make their income is not correlated with what makes a good clinician.