This very law can at some point in the future be turned against the very same people that support it today.
If a law like this can be justified, then any law forbidding any type of speech can be justified.
Which is why our founding fathers gave us the first amendment.
Austria certainly has the right to pass such a law, and fortunately, we have the right to be critical of the law (with out it being assumed we support the views of the person being prosecuted).
That is what reasonable people do.
I know we all get frustrated with those that disagree with us from time to time, and secretly wish they would just shut up, or there was a way to shut them up, but preventing people from expressing their views does not mean you won them over to your side.
This type of outside force tends to keep people quiet on the surface, but it is like a boiler with the safety valve plugged up. Instead of the pressures being released safely, it builds until there is an explosion.
The best example of this is the old Soviet Union. They had plenty of laws that prevented people from speaking out until eventually it exploded.
And I get the "slippery slope" argument, especially regarding freedom of speech. All dictatorial regimes have used censorship and propaganda as methods to control their citizens. But lets keep our perspective and stay with this one example rather than entertaining all hypothetical situations.
The Austrians have the right to decide this for themselves. If the Austrians want to jail Irving for what they consider to be inciteful speech or hate speech, then it is their right. Even we place limitations on the freedom of expression. My only hope is that we limit freedom of speech on a case by case basis and for reasons more substantial than just not liking what someone has to say.
You can't slander. You can't scream "FIRE" in a crowded theater. You can't lie in advertising. All violations of freedom of speech. That the holocaust happened is an objective FACT. That it was pure evil is a subjective opinion. To me, lying is a greater offense than having an opinion I find offensive. In all but extreme situations, we are free to have and express offensive opinions, but not to lie about facts.