I believe part of what we "assume," is the various Confederate States adhered to every edict that came down from the Richmond Government, especially concerning replacing troops in the field, as the war began to go badly, and the number of volunteers began to dwindle.

June 1863, Series II, Volume VI (to read the entire report)

Lieutenant-Colonel William H Ludlow (Agent for Exchange of Prisoners / 73rd New York Volunteer Infantry)

And more recently the Confederate legislature of Tennessee have passed an act forcing into their military service (I quote literally) all male free persons of color between the ages of fifteen and fifty, or such number as may be necessary, who may be sound in body and capable of actual service; and they further enacted that in the event a sufficient number of free persons of color to meet the wants of the State shall not tender their services, then the Governor is empowered through the sheriff's of different counties to impress such persons until the required number is obtained.