Why was it legal to have a muslim slave in Islam?

It says in The Kuwaiti encylopedia that the purpose of slavery is to punish the disbelievers for not submitting to God. It also says that slavery is humiliation for the enslaved person. Please see: - https://shamela.ws/book/11430/13765#p1 - https://shamela.ws/book/11430/13763#p1 If this is the purpose of slavery, to humiliate those who do not submit to God, then why does Islam allow a muslim to be a slave? - For example a non muslim slave may convert to Islam, but from what I understand it is still not a leal obligation to free him/her; or a woman can give birth to a slave child who can be raised as a muslim slave. There is a hadith that a fellow muslim cannot be mistreated, so I am asking for clarification.