Slavery isn't defined as an inherently legal practice, it simply was legal in some places at some times.
The secondarily linked story does also talk about local law enforcement being involved, so that is at least condoned "in a way", not "not in any way".
The conversion of something to property is almost always a theft of some sort.
Humans held and treated like property will henceforth not be considered slaves if their country has laws against the practice. When asked how he came upon the genius solution to the century-old problem, marknutter nodded at the portrait of former President G.W. Bush hanging above his fireplace. "I was inspired by President Bush... I mean... enhanced interrogation... genius!"