This is actually a very interesting question to consider when we relate it to the character of Huck and how he develops through the story. What we see from start to finish is the gradual development of Huck's own sense of right and wrong and his increased determination to stick up for what he knows to be right. This incident, as it comes in the middle of the book, displays a Huck who is all to ready to take the quietest option rather than doing something as defiant as denouncing the Duke and the King as liars Note what Huck says to us to explain his position at the end of Chapter 19:
It didn't take me long to make up my mind that these liars warn't no kings nor dukes at all, but just low-down humbugs and frauds. but I never said nothing, never let on; kept it to myself; it's the best way; then you don't have no quarrels, and don't get into no trouble.
At this stage in the novel, Huck's development and maturity is still at the stage where, as perceptive as he is, he still wants to take the path that will result in "no trouble" and no conflict rather than sticking up for what is right and what he knows to be true. This is of course something we see later on when Huck decides to act to impede the plans of the Duke and the King when they impersonate the Wilks brothers.