Residency supervision is a critical time for counselor identity development. A large part of the supervision process involves mentoring and conversations around advocacy and professional roles. For this reason, it is important that the mentor in the relationship share the same professional identity. While I value interdisciplinary collaboration very much, that will inevitably happen on treatment teams and in the work place. Those conversations will be useful - and having a supervisor from the same discipline to discuss them with is the best case scenario. Psychology and Social Work do not allow Counselors to supervise entry level professionals for these same reasons.
It is not a sign of anything beyond a desire for these young professionals to be clear about who they are as a part of a distinct profession. Being clear about professional identity allows us to bring our different strengths and skills together as working professionals.
There is a reason why we are distinct professions with different requirements for training and licensure.