It seems that King Affonso may have allowed the Portuguese to enslave his subjects at first without knowing the possible repercussions. As the saying goes, "if you give an inch, they'll take a mile". In addition, the Portuguese began kidnapping the people, including noblemen. After seeing all that was happening, King Affonso must have came to a realization that there was something wrong, leading to his request for regulations.

I think that King Affonso was ok with the slave traders taking the lower class and the prisoners of war, because those people were worthless. The slave trade grew rapidly and more slaves were needed, the Portuguese ravaged the society and took what slaves they wanted. King Affonso opened a Pandoras box.