They've always liked slave labor because they see work as a punishing thing to force people to do, and hard, painful and dangerous work is what you force on people you don't like(/judge to be immoral).

Increase the demand for productivity until the workers are sufficiently suffering (according to your personal idea of how much a worker should suffer). Now you're their shepherd, putting them to work. You've taken personal responsibility to ensure they don't have idle hands to do the devil's work or whatever. The point is that you are good for forcing them to do this.

This, I think, is the foundation of the Christian work "ethic". Which essentially is to voluntarily punish yourself woth your work like this, to save your boss the trouble. Leaders love this religion for some very good reasons.

The profit motive isn't irrelevant to the equation at all, of course, but good profits aren't necessary for them to desire the continuation of punitive labor.

Same is actually true for most fossil fuel production now - only has profits because of subsidizing.

The folks profiting spend part of that profit to pay for politicians to continue the cycle.