Not a single country allows unlimited freedom of expression. Every single one, including the US, has substantial limitations based at least on situations where that speech harms others (e.g. slander/libel/defamation/incitement laws).

In that respect laws specifically targetting a very specific set of lies, that also happen to be defamation of a large group of people (and so potentially actionable based on other laws in many cases anyway, including in many cases in countries without laws against Holocaust denial) is one of the most targeted, limited such restrictions around.

Holocaust denial is in the same vein of "defamation" as claiming that the Bush family are secret reptilians from outer space or that the moon landing didn't happen--there's no risk of confusing a reasonable person with these claims, nor do they especially fall under the banner of normal defamation.

The entire lesson of the Nazis is that awful things can happen when the State appoints itself as the unquestionable arbiter of truth and refuses to allow open discussion about its own dogma. Freedom of expression simply does not exist when governments have the power to rule facts into law by fiat and render them illegal to question or dispute.