Leaving aside the non-member nations that did not pay Celestial Gold, free cities under the World Government like Water 7 were, without exception, territories directly under the World Government. The Celestial Dragons had no need for another layer of middlemen to take a cut and reduce their profits.
Member nations like the Sorbet Kingdom, which only needed to pay protection money—Celestial Gold—were different. The World Government had limited control over them, making the existence of nobles, a class defined by bloodline, especially necessary.
On one hand, it was like how the great powers on Earth chose to preserve China's decaying Qing Government during the Late Qing Dynasty: those without the people's support were easier to control. On the other hand, the Celestial Dragons, who styled themselves as World Nobles, felt a greater affinity for their own kind. To a certain extent, this also became a test of obedience aimed at the citizens of member nations.
If a nation could do without its own nobles, then why could it not do without the nobles of the world?
The thought that the changes in the Sorbet Kingdom might inspire more nations to follow suit made even the Five Elders uneasy. They sent Marines to support the fleeing nobles in returning home and overthrowing Bartholomew Kuma's rule.
Unable to resist, Bartholomew Kuma could only take to sea and become a pirate. His pirate crew was especially ruthless toward nobles. Unlike their peers, who generally only used captured nobles to demand ransom, they killed few but made a massive impact. Thus, the name "Tyrant" grew ever more famous.
As for the princess he had raised, Jewelry Bonney, the restored nobles had originally intended to install her as Queen to serve as their figurehead. Yet on the eve of her coronation, the little girl vanished