Slavery requires coercion. If it's by choice it's different but there are other laws under which this falls that prevent it to avoid abuse. For example in the U

Slavery requires coercion. If it's by choice it's different but there are other laws under which this falls that prevent it to avoid abuse. For example in the US, you cannot employ a intern for free that does work your company might profit from (not a lawyer so I might have that slightly skewed).