There is no prohibition to question alleged facts. However, to question a sum of historical facts which are proven and documented with the intention to defend the Nazi regime or express support for it is criminal. Dumbness isn´t a crime, but intentional misleading of others/spreading Nazi propaganda very well is.
Every new generation is a blank slate that must be educated all over again.
You and I are of the generation that has studied the sum of the historical facts and have convinced ourselves of what really happened and can defend our position.
However, once new generations are told, "This is so because we say so and you are not even allowed to question what we say or you will be jailed", that passes into the realm of dogma which threatens the credibility for future generations.
For example, was the beginning of the "The Diary of Anne Franke" actually written by a 13 year old girl? (That's a trick question that requires some research in order to arrive at the correct answer.)
To criminalize such questions casts doubts on the authenticity of the entire work in the minds of future generations and defeats the work itself.
Though I am against everything the Nazis stood for, holding an opinion, however ridiculous or insane, should never be a crime. I believe in freedom, even for those who would advocate at the top of their lungs that which I would spend a lifetime protesting at the top of mine.