In some ways, Nemo has incredible freedom, yet he's confined to a cigar-shaped metal boat for most of his days. Aronnax, Ned, and Conseil enjoy a similar kind of freedom as Nemo while they're his captives: there's nothing to stop them from exploring the ship or the wonders of Nemo's world. But they must never leave the Nautilus. Confinement within freedom… This kind of contradiction is the name of the game in 20,000 Leagues Under the Sea. For example, for Aronnax, the freedom to study comes at the price of the freedom to leave. And he has serious trouble deciding which is more important.
Verne shows us that freedom is entirely subjective; while Nemo and his crew may enjoy the vast pleasures of remote undersea worlds while aboard the Nautilus, they can never truly escape the ship.
By forcing Aronnax, Ned, and Conseil to stay on board his ship, Nemo undermines his own attacks on tyranny and despotism. His rules are just as arbitrary as any other ruler's.