advocated might have been the wrong term.... acknowledged might be more accurate. The fact is, there were slaves all through history... and ALL slavery is man-stealing, in one form or another. I don't know of anyone that volunteered to become a slave.
Indentured servitude is a different thing.... many times it IS done with the servant's permission. Sometimes it was a penalty for crimes committed... I've read many articles that say that indentured servants in the US were treated worse than slaves were.... slaves were bought, so most slave masters did not want to unduly damage their "property". Indentured servants were not paid for by the masters, so they really did not care about the welfare of the servants...
I'm certainly not an expert on this topic, but I have read some on it....