Despite what the Dansker has told Billy, he can't imagine suspecting the pleasant-seeming John Claggart.
The narrator spends a few paragraphs defending his myopia (short-sightedness).
As he explains it, Billy is too young to have been taught by experience and is just so GOOD that he doesn't understand how the minds of bad men work.
Plus, he's a sailor, and "The sailor is frankness, the landsman is finesse" (16.2).
Life is much simpler for the sailor. He doesn't constantly have to brush against other men, both in his business and his personal life, whom he would have reason to distrust. Whereas others gradually take on "a ruled undemonstrative distrustfulness," the sailor remains essentially innocent and light of heart (16.3). Or at least that's the way our narrator presents it.