I've read shield hero (the manga) and even though the protagonist buying a slave makes sense at the beginning (he's paranoid and can't trust anyone) and the middle (the shield hero is the hero of slaves, slave party members have a stat boost), I'm still baffled that the hero has no intention destroying the concept of slavery that he knows exists and oppresses two of his party members.

Worst of all, the slave trader is a recurring friendly character.

I would love to dig deep into this, but honestly? Most likely originates from creators trying to pander to lonely, antisocial men by saying "heres a world where you can literally BUY a gf!"