Why is it socially acceptable to say you hate white people?

My counsin who's in 5th grade had been the victim of bullying from other children, black children, because of her color. Calling her a cracker, privileged bitch, white scum, and going as far as to spread rumors about her online. I first asked if she said anything bad to them, she said she doesn't know. I asked if she said the N word, she didn't even know what it was or what it meant. So I called the school wondering what in the hell is going on here? While on the phone it turns out the teacher had mistaken the bullying for "They were just messing around with her, thats just how they make friends". Of course this was not acceptable, I wanted to know what made these girls mistreat my cousin, not some half assed lie. I asked again, what did my cousin do to those girls? And the subject matter changed to "well black people are always mistreated, and it's because of what the whites did in history-" Again, how did this excuse the bullying exactly? I was getting no where with this woman and hung up. Now ever since Covid hit, my cousin has moved in due to her family's financial troubles, she also needed reliable internet ever since remote schooling came into the scene. So.. what's the big deal here? I sent her back to school the day after and had to ask the girls why they were mistreating her. It was just so random and out of nowhere. And so I come to pick her up from school and she's in tears. Telling me she hates school and never wants to come back. She's usually very happy and bubbly, even last year when her parents were arguing and fussing the summer the pandemic hit, I usually babysat her when they were at work or busy. So after a few hours I got her to open up to me about what happend and those girls who were bullying her, told her they hate white people and told her should die, that all white people deserve to die. Another call to the school and no change from before, going as far as to say it was ok to say thet hate white people because of history.

So I notified her parents and had her take schooling online only. It's been about a month since then and she's calmed down a bit. But I'm still pissed off, So I ask, why is it ok to say you hate white people? I usually avoid social media due to the toxic community in every platform. But I needed answers but found nothing. So, if it's ok for a black person to talk about hating a white person yet it's bad for a white person to say they hate a black person. This is 2021 now, not the 1800's, not the 1940's. Using history to justify mistreating someone just because of their skin color.
So, as controversial as this is, why is it acceptable?