This clause echoes the transgression described in Revelation 17:2. The “her” (prostitute) was Babylon (any civilization, city, state) that enticed people to find their sense of well being not in God but in any created thing (be it money, health, sex, power, etc.) The “kings” have fallen for the prostitute’s enticements, are in bed with her, and so worked along with her to lure the “little” people (specifically, God’s people) to join in spiritual adultery.
9 And the kings of the earth, who committed sexual immorality and lived in luxury with her, will weep and wail over her when they see the smoke of her burning.