Why did Hans Junior accuse his father of cowardice? What was the 'mistake' Papa Hubermann had committed?

Hans Senior is not a Nazi, and he blew his chance to join the party for painting over anti-Jewish slurs. The father and son argue fiercely; Hans Junior accuses his father of being disloyal to Germany and asks why Liesel isn't reading Hitler's book Mein Kampf. Hans Junior then calls his father a coward for doing nothing while "a whole nation cleans out the garbage and makes itself great" and storms out. Check it out,

http://www.gradesaver.com/the-book-thief/study-guide/section3/