The only way a slave can exist back then was if prisoners of war have not been ransomed, after ransom demands have been made, and they are not people of the book (Christian or Jew). If they don't accept Islam then they became slaves, if they did then they where free. Also this is 14 hundred years ago and isn't applicable to today society. Slavery was an issue that existed everywhere at that time and Islam needed to lay down rules to deal with it. Outside of that legal slavery never existed in Islam. Islam abolished racial slavery. Even then the children of slaves could not become slaves. With Islam came many rights and better treatment for slaves who at the time had no rights. Don't compare that to American slavery which was the worst manifestation of slavery that ever existed.
There are many hadiths encouraging freeing of slaves and the great rewards one would revive from Allah. The companions used to look everywhere for slaves to free, until slavery was abolished by Islam in this way. I have explained this before. Many of the Caliphs of the Ummayd and Abbasid dynasties where sons of Slaves. Islam abolished slavery 14 hundred years ago. If Arabs practice slavery then they are disobeying the teachings of Islam, it had nothing to do with the teachings of Islam. Slavery exists and has existed until recently in the Americas, Asia, Africa, Europe and many other places, I don't see you guys accusing other religions of being pro slavery.