Why did Papa slap Liesel?

Hans meets Liesel at the church steps, and she asks him if her mother is a communist. Hans lies and says he does not know. Liesel asks if Hitler took her mother away, and Hans, finding it impossible to lie, says yes. Liesel says she hates Hitler, and Hans, worried about what to do, slaps her and orders her never to say that again. Hans forces Liesel to do a proper Nazi salute and say "Heil Hitler." Hans is really protecting Liesel in public.