This is true for MUSLIMS only. If they convert, they're free. This is why for generations slaves were not taught about Islam deliberately
Some are angry that slavery is now forbidden on Muslim lands when Allah has allowed it. Including American Muslims. https://www.google.ca/amp/s/www.washingtonpost.com/amphtml/news/answer-sheet/wp/2017/02/17/georgetown-professor-under-fire-by-conservatives-for-lecture-about-slavery-and-islam/ …
