People often say that there is no such thing as an honest merchant, and merchants often resort to unscrupulous methods to make money. Ironically, however, merchants have their own set of morals, or rather, rules of the game.
As long as you're not planning on a one-shot deal, business people know the importance of honesty and openness because they determine whether you can gain the trust of other merchants and get opportunities for collaboration.
Xiao Ming's straightforward revelation that he was making the call for his own reasons was based on this principle.
If he hadn't said anything, Zhou Jicang would inevitably feel resentful when he later discovered that Xiao Ming's goal was to take control of Dabang, and although it wouldn't harm Canghai, Zhou Jicang would feel that Xiao Ming was not a person to be closely associated with.
In short, when doing business, having a long-term vision is very important; you can't just focus on the small immediate gains.