The need for such a statement seems strange, for - in the previous pasuk - the Sanhedrin proclaims that, "Our hands have not shed this blood, nor have our eyes seen (the murder)." (ibid. v.7) Rashi further explains (from Gem. Sotah 46a) that the Sanhedrin's statement refers to the fact that they did not neglect the victim while he was in town; they did not fail to offer him food or escort (such that he took to the road without security or provisions). It is clear from pasuk 7 and Rashi that the town did not in any way contribute to the death of the victim; why, then, does pasuk 8 invoke forgiveness for the event?
I think that the answer is that although there is no individual culpability, there is an underlying communal responsibility which was amiss. These two levels of responsibility bind each Jew and the Jewish people as a whole. Thus, any wrong which occurs under our auspices, while not attributable to known single factors or persons, is - by definition - accounted to the community as a separate "corporate" entity.
This notion is deeply alluded to by the anonymity of the victim (v. 1), the unknown site and circumstances of his death (v. 2 with Targum Yonasan ben Uziel), and his unknown fate, had he lived (Rashi on v. 4). We cannot pin-point anything about the crime; it is a vast mystery. So, too, the larger communal entity, which is undefined as to individuals, is held accountable.
When someone is victimized under our shadow, something went wrong and we must repent for it; when wrongdoing occurs in our midst - even when no one seems to be culpable - we must take communal responsibility and corrective action.