> What was wrong with I'm sorry [that] you're leaving as a
> translation of the J here?

Nothing that I can see, but it sounds as if you think this sentence has been changed - it hasn't. It's one of the original Tanaka Corpus sentences.

I deleted my original comment: the explanation was slightly faulty. I would simply say the header sentence is not English I can understand. It is all to do with what uses of should which are standard and acceptable [to me/you]. As it stands it is not only confusing but is definitely different from the Japanese, which I would read as it os "It is too bad... or even "It's terrible" [the latter would be slightly over the top to most of us though, I suppose]... that you are going." "I'm sorry that you're leaving [here]." seems to be a good and transfers well across all the Englishes:
"I'm sorry you're leaving us" always seemed the accepted phrase up north even when there was no discernable "us"

I don't disagree with your correction, I just think you were over-egging the pud with your reaction. There are probably, by my estimate, 5,000 more sentences from the original Tanaka Corpus that need correction just as badly. And that's after eight years of fixing and deleting examples from it.