Parent poster a-french-anon may be wrong or at least is making unsubstantiated wishful claims about costs and benefits - "the errors would be extremely rare" - would they really? And would they be evenly spread over in-groups and out-groups?

But at least the question "how is that acceptable?" is in fact a question of a moral nature. It's unacceptable, but it is unacceptable because it is immoral.

How is any error rate, no matter how small, acceptable when it comes to locking people up for the rest of their lives?

While I don't like the death penalty I don't think it's that different from a very long sentence. I don't think it makes sense to say that any punishment needs an absolutely perfect error rate.