They're mad that people generally have a positive view of Japan and it's media, while having a heavily negative view of their own bullshit. It destroys the narrative that people don't like them because they're bigots, and shows instead that they just suck ass at creating interesting and compelling media. One Piece specifically has tackled pretty much all of their pet issues in depth, but has done so in a way that isn't offensive to everyone else. Hell, one of the main protagonists is based pretty much directly upon gun loving hillbillies. Oh, and Bonney's father here is the shining ideal of a good, strong, Christian man who sacrificed everything for his daughter.