Tolerated? Some welcomed it and some actively fought several wars against it and lost. Many tribes conducted some kind of economic transaction that traded land for something else.
> 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.
Which is functionally the same as
> slavery or involuntary servitude, 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.