I mean, "so what" seems ok until it's more than a thing a random hn commenter heard from someone when they volunteered at a prison that one time.

So what doesn't call in to question the veracity of the statement, it implies that the situation described doesn't matter. To me it sounds like "Well, the problem is hard, so we should just accept that some people may die and live with it" as opposed to "we don't know how common that is" or "the alternative is worse and here's why".

My objection is to what I see as a cavalier attitude towards the injuries and deaths of those we've incarcerated, which I think we have a clear duty to try to alleviate. We are legally required to not torture and or injure them, and I don't think turning a blind eye towards what happens in the environment we force them to be in diminishes our responsibility with respect to that.

I think you're taking a particularly uncharitable reading of GPs "so what". I read it precisely as asking the other person to make the value tradeoff argument.

Perhaps, but Merriam-Webster defines the idiom as meaning "should not be found objectionable", which squares with how I interpreted it. You're right that it's possible it wasn't meant that way though.