how did narrator and harris get blackmailed for traspassing ??(chapter 8)

Harris and J. stop to eat lunch by the side of the river. A man appears and accuses them of trespassing, threatening to report them to the landowner. Harris – a large man – physically intimidates the visitor until he leaves. J. explains to the reader that the man was expecting a bribe, and most likely did not work for the landowner at all. He adds that these attempts at blackmail are common along the banks of the Thames, and that tourists should avoid paying people who do this.