The World Government is an oligarchy. Which means the rich nobles at the top make up whatever rules they see fit. In actuality the nobles themselves have very little real power but because of their lineage they're afforded a lifestyle no one else is an effort to keep them happy and out of the actual governing body's way. Just as merely hitting one can result in an admiral being personally called in to deal with the perpetrator. Currently the only legal slavery is of pirates and fishmen and mermaids because it's considered their lives are forfeit so long as they have a bounty on their head and therefore it doesn't matter what is done to them.

You know my feelings on Doflamingo. He makes his fortune precisely through the slave trade and his shichibukai status gives him government sanction to do things not even slavers could normally get away with.

I despise it. Even for pirates.

But one thing at a time. Justice is the most important value especially in an oligarchy because it affects how soldiers and citizens help or change the way nobility is run.