They're terrible people, right? But if I say to you, "Stay away from Grace, she's a kitten-eating monster," you're going to assume I'm joking. That's hyperbole. That's actually a terrible thing to say about someone.
So everyone in Group A doesn't have to worry about rumours that they eat kittens, because seriously? Who does that? You must be joking.
But if anyone does say, loudly, "Hey, these people seem to be eating kittens," they can say "Well, you eat BABIES!"
Because then people who don't know them are going to go, "Right, these people are throwing hugely exaggerated insults at each other, got it." Because these look like the same insult, if you don't know the history. And even if you try to show them the history, well, you must be blowing it out of proportion, right? No one eats kittens. Not in our society.
The fact that one of these insults is in fact true - people have actually been eating kittens - and one is false - no one has eaten a baby - may be ignored.
And the second person now has to fight against accusations of baby-eating - which they do care about, because they don't want to be seen as a horrible person. They don't eat babies, and they're probably really upset that someone said they did. And a few people are going to be saying "Well, they must have a reason for saying you eat babies, and you did start it by saying that they eat kittens..."
So no one wants to accuse actual kitten eaters of eating kittens, and if you do, the first thing that does is cast suspicion on the one doing the accusing.
Anyway, if you're wondering where this is all coming from, someone in the Sherlock fandom ate a kitten in public and then tried to distract everyone by going "THINK OF THE CHILDREN!" If you really want more information, it's here.