Why do all the boats on the inland rivers fly the national flag?

This is a legal requirement.

Article 4 Ship registration shall be carried out in accordance with the relevant ship registration regulations of the People's Republic of China, and only ships that have obtained the nationality of the People's Republic of China may fly the Chinese flag as the flag of the ship's flag state.

Article 5 Except for the circumstances specified in Article 8 of these Measures, the following Chinese ships shall fly the Chinese flag every day:

(1) Ships of gross tonnage and above;< /p>

(2) Ships sailing in waters outside China’s territorial waters and Hong Kong and Macao;

(3) Official ships.

Article 6 Foreign ships entering the People's Republic of China and domestic waters, ports, and anchorages must fly the Chinese flag every day.