You have a point, though West Virginia succeeded from a Confederate State that was, at that time, not part of the Union. I suppose it is possible for a State to fracture and agree to become two States, but not for counties to do so without the consent of the State.
The Confederacy was never officially recognized, so legally, it didn't exist. A particularly clever lawyer, one Benjamin Butler, thus saw the loophole, and declared that any slaves "captured" from the Confederate states could be called "contraband," since their labor was of value, and would belong to the Federal government, who could free them at will.
Is there a prohibition clause in the California constitution?
As an aside, one newspaper editor said that "South Carolina is too small to be a country, and too large to be an insane asylum." I suspect these 12 counties are in the same position.
This kind of sophistry is what keeps the legal profession in business.