One way to answer your question is to consider the role of Major de Spain.
Major de Spain is prejudiced against the Snopes from the beginning. The way they look and speak is enough for de Spain to dismiss the whole family as unimportant, to "take them for granted" particularly Abner. De Spain is aristocracy (though of the Southern variety) while the Snopes are white trash. He may not "ask" others to accept his opinion of the family; he simply assumes it will be so.
Snopes may not do anything to challenge de Spain's opinion, but he certainly acts in a way de Spain would never thought possible: that is, a person he held in such contempt could actually be smart enough to do him harm.