They are not? Look it up. There are several people in prison RIGHT NOW for questioning aspects of the Holocaust.
Do you think there is a difference between questioning the figures of the Holocaust and denying it altogether?
Those who were imprisoned were not sent to jail for "questioning". They were jailed for denying it ever happened and for directly manipulating and misrepresenting the evidence, or ignoring it altogether to further their own agenda. Irving is a neo-nazi, anti-semitic racist who not only denied the holocaust, he also misrepresented and manipulated historical evidence to support his cause. Citing this particular individual, at least, will result in a great loss of credibility in this debate.
There are many historians, qualified historians, who study the holocaust in detail and who do question the exact figures stated and who research it to the finest degree of detail. They are not arrested. Do you know why? Because they present their findings as they are and do not attempt to misrepresent or ignore evidence to further their own personal beliefs about the Nazis and the Jews. When new evidence presents itself, they revise what has already been written about it in the past. In short, they are researchers and historians who are attempting to study a horrible part of human history.
David Irving and his ilk are neo-nazis with their own agenda and will do what they can to not only misrepresent the data, but have and will suppress anything that contradicts their own personal beliefs and cause.
So what is the difference between questioning the Holocaust and denying it ever happened? One researches and constantly updates and revises what is already known without any misrepresentation but only based on facts and the other manipulates the evidence so that it is completely misrepresented to further their own agenda.