Banning slavery caused dead-weight loss. Market clearing price for commodified labor is minimal sustinence.
Banning slavery eliminated massive dead weight loss. Slavery used violence to force labor from people below the clearing price - it compelled them to work for free.
All liberals should oppose slavery. We should want free people, free markets, and a generous social safety net for all.
To be clear: I'm not for minimizing deadweight loss because free markets are horrible for humans. Free markets don't care about human suffering.
Regulated markets can be OK, but then you shouldn't talk about free markets or deadweight losses.