I think you fundamentally misunderstand how much my company cares about their employees dying, which is 'only as much as it's inefficient to have to train new ones all the time'.

[If none of the people making the decisions are the ones who get killed, then there's no risk whatsoever.]

But to answer your question, their brain activity generates a substance called enkephalin, which can be used as a power source. It spikes considerably when they're interacting with humans in some way, which is why just leaving them alone in their cells isn't profitable.