> My theory is that the government made them legal to appease Southerners after the Civil War.
It didn't; this is indisputable, because they existed and were legal, both in and out of the South, before the Civil War.
You could argue that they were not outlawed at the time chattel slavery was in order to mitigate the imposition on the South being done by the Civil War amendments (13-15), but that's implausible for a number of reasons, but most notably that being punitive was major feature of th Civil War Amendments, the cooling to punitive measures and accommodationist approach to the South didn't take long to appear after they were imposed, but it was a later thing.
> the evidence in terms of the Congressional record of the time
It didn't; this is indisputable, because they existed and were legal, both in and out of the South, before the Civil War.
You could argue that they were not outlawed at the time chattel slavery was in order to mitigate the imposition on the South being done by the Civil War amendments (13-15), but that's implausible for a number of reasons, but most notably that being punitive was major feature of th Civil War Amendments, the cooling to punitive measures and accommodationist approach to the South didn't take long to appear after they were imposed, but it was a later thing.
> the evidence in terms of the Congressional record of the time
Specific citations would be welcome.