Why did Papa Hubermann slap Liesel?

Papa Hubermann slapped Liesel for saying she hated the Fuhrer. He wanted to make it clear that she could never publicly say such a thing.... that those words were dangerous.

“I knew it.” The words were thrown at the steps and Liesel could feel the slush of anger, stirring hotly in her stomach. “I hate the Führer,” she said. “I hate him.”
And Hans Hubermann?
What did he do?
What did he say?
Did he bend down and embrace his foster daughter, as he wanted to? Did he tell her that he was sorry for what was happening to her, to her mother, for what had happened to her brother?
Not exactly.
He clenched his eyes. Then opened them. He slapped Liesel Meminger squarely in the face.
“Don’t ever say that!” His voice was quiet, but sharp.

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“You can say that in our house,” he said, looking gravely at Liesel’s cheek. “But you never say it on the street, at school, at the BDM, never!” He stood in front of her and lifted her by the triceps. He shook her. “Do you hear me?”