I think this argument would have to conclude that training any RL agent at all is unethical, since updating its weights 'away' from some stimulus could be considered pain.
I think what I'm getting at is that there are different types of 'pain', some that we'd consider ethical and some that we wouldn't.
Constantly subjecting a person (or some abstract simulation that responds like a person) to the equivalent of continuous bodily pain would be deeply unethical, but, say, giving a person clues towards solving a puzzle would be considered less so.
Overall, I think you're right though, if we somehow discovered we've created simulated people (or sentient beings) then we probably shouldn't use them to solve arbitrary problems.
RL agents are exposed to millions upon millions of stimuli, almost all of which will be 'painful' at least initially (according to the websites definition of pain). I think for children negative stimuli are not all painful, pain is a certain small subset of negative stimuli that damage is being done.
I think children are constantly experiencing rewards and punishments of a sort analagous to reinforcement learning. Until a certain age their brain is hardwired to want to please their parents, so they pay a lot of attention to the parents' faces, and find reward in smiles and find punishment in frowns. After that, they are driven by a certain amount of ego, and find reward in self-accomplishment and find punishment in being told what to do.
Constantly subjecting a person (or some abstract simulation that responds like a person) to the equivalent of continuous bodily pain would be deeply unethical, but, say, giving a person clues towards solving a puzzle would be considered less so.
Overall, I think you're right though, if we somehow discovered we've created simulated people (or sentient beings) then we probably shouldn't use them to solve arbitrary problems.