Everyone knows it's fiction because watching it sends them to google/wiki/youtube to find more info on the real thing, lol.
And guess what? That stuff is basically confirming what they saw on the show. The Diana Netflix doc In Her Own Words? All the rest of them? Come on.
They just don't like that this is reminding everyone and teaching a new generation how much they hate Charles and Camilla. Well, screw them. They deserve it, imo. Charles is an asshole. He was never personally popular, so the best thing he could have done to ensure the survival of the monarchy after Elizabeth died was to marry a woman the public adored and wanted to see as Queen. That was his best case scenario, and amazingly enough, that's exactly what he wound up with!
He should have embraced her popularity and used it to his own advantage instead of being the sniveling, spoiled, resentful little prick he was. Diana would have done anything for him, all he had to do was be nice to her- he probably could have even kept Camilla as mistress if he'd just shown her some love/affection and not rubbed it in her face. But he couldn't even do that!