After WWII, there was a massive backlash against german collaborators. Everyone even slightly suspect of being pro german was at risk of being looted, tortured or killed by their furious neighbours.
Mob rule had to be replaced by lawfull governement, and this required appeasing the mob. Part of it were dishonest trials, a blind eye against some revenges. A very small price to pay was forbidding the denial of the crimes that the Nazis had done.
I am not proud of this part of our history but it was probably necessary.
However, we are a few generations further now and a lot of relevant people are dead now. Maybe the time is ready to review this rule. Discussing this stuf is especially important as we seem to remake some of the errors that led to the war