Because Hunter & the big guy told them the Three Gorges Dam was a target of the US Military if China messed around too much
I expect the chicoms didn’t have to hear it from the Bidum Gang that the 3G might be a target. Never mind the main dam. Those locks are in range of a volley of Tomahawks and if even one door got hit chyna would never be the same. Another reason for Xi to be polite. And remember , they’ve been a bit testy over the USS Kentucky parked at a dock in South Korea. And the Kentucky can open those vertical launch tubes and fire while dockside-—should somebody get snippy.