Why is it that in the early years of Islam, after the battles that Mohamed himself participated in...?

...even women were taken as captives and not freed? Juwayriyya bint al-Harith was a captive taken after the expedition against the Banu Mustaliq and then Mohammed took her with him and married her. Safiyya bint Huyayy was also a captive taken after the Muslim army conquered Khyber, she was initially given to another Muslims as part of his but when Mohammad saw her, he apparently fell for her and threw his cloak on her as a sign that she was his chosen one. Now, I want to ask; why didn't the world's most noblest man let go these ladies after killing their families instead of taking them as his slaves? And if you're gonna say that Mohammad wanted to give them protection so let me ask why didn't he took them as his daughters instead of his wives? Was he not taking advantage of these women?