According to Nick Craver, the reason for not switching away from the current behaviour is because it performs much better than the alternative. I assume this is because the badge process for Unsung Hero checks the denormalized post score from the Posts table, instead of going to the actual Votes table.
However, I entirely agree that this approach would make far more sense if it were technically practical. In spite of that, I was hoping to point to a query that suggested this wasn't really an issue anyway, but it seems like quite a few usersquite a few users with the badge have one or more answers that had an even +/- split as of the last Data Explorer update (December 22nd) and were posted before the user received the badge.
To be clear, this does not mean that those posts necessarily contributed to them being awarded Unsung Hero, since the query does not factor in the date of each of the votes. Additionally, it's entirely possible that the users had enough other zero-score answers at the time, or would have gotten more shortly thereafter, that the presence of these answers is irrelevant. Still, I imagine it is likely that for some users these answers played a part.