At least in the US we have this statement in the Constitution: > Neither slavery nor involuntary servitude, except as a punishment for crime whereof the party

At least in the US we have this statement in the Constitution: > Neither slavery nor involuntary servitude, except as a punishment for crime whereof the party shall have been duly convicted, shall exist within the United States, or any place subject to their jurisdiction. There is no such supporting language for calling imprisonment "kidnapping", whereas compelling labor from prisoners is explicitly identified here as your choice of "slavery" or "involuntary servitude".