If I was a coach I would be disappointed and upset if my team did this, however the initial hit from the linebacker on the white team was not illegal. There was no whistle. The O-line was standing straight up complacent, the QB waited about 1.5 seconds to take the knee.
Not hitting them on a kneel down is more of a courtesy and unspoken rule IMO.
Again, I don't condone the whole thing, but I think the black team was more in the wrong initially. The white team just kept it going longer.
That being said, if I was on the black team and my boys didn't back me up after a hit like that I'd be upset with them.