Muhammad declared that not all slaves should be set free...

Consider the following hadith, from Sahih Bukhari:  "Narrated Kurib: the freed slave of Ibn 'Abbas, that Maimuna bint Al-Harith [One of Muhammad's many wives] told him that she manumitted a slave-girl without taking the permission of the Prophet. On the day when it was her turn to be with the Prophet, she said, 'Do you know, O Allah's Apostle, that I have manumitted my slave-girl?' He said, 'Have you really?' She replied in the affirmative. He said, 'You would have got more reward if you had given her (i.e. the slave-girl) to one of your maternal uncles.'"
Our Debate Question is simply "Why would Allah (who grants 'rewards') want this slave to continue to be enslaved, rather than to be immediately set free?"
Now consider this hadith, from Sahih Muslim: "Imran b. Husain reported that a person who had no other property emancipated six slaves of his at the time of his death. Allah's Messenger (may peace be upon him) called for them and divided them into three sections, cast lots amongst them, and set two free and kept four in slavery; and he (the Holy Prophet) spoke severely of him."
2nd Debate Question:  "Why does Muhammad prefer most of the slaves being kept in bondage rather than being set free, when the slaves are not even his to begin with?