Why does sheriff Tate insist that bob reel fell on his own knife, even when Tate knew it wasn't true

In Chapter 30, Atticus is trying to uphold the law by demanding that Jem be brought to trial for the crime of murdering Mr. Ewell, not realizing that the sheriff is trying to protect Boo. As seen before in the case of the Ewell's, who are allowed to hunt in season, the law must be bent in order to protect certain people; in this case Boo needs protection. When Atticus understands the sheriff's motivation, he relents, realizing that it is in everyone's best interests to allow Boo to unofficially punish the Ewell's for the crime of trying to send Tom to his death. By this point, the "mockingbird" theme has already been made clear, but this chapter rehashes the idea that the innocent should not suffer in the hands of the powerful.