African Americans are disproportionately inprisoned because African American commit crimes at a disproportionately higher rate that other ethnicities.
That is flat out false. There's a great deal of scholarship done on this and what it shows uncontroversially is that when a white person and a black person commit the same crime, the black person is more likely to be arrested. In the less likely scenario where both are arrested, studies show that a black person is more likely to receive a longer jail sentence, if the white person would even be incarcerated at all. And what is the reason why black people are arrested for crimes that white people are also committing but are not being arrested for? Because black communities are under greater surveillance, largely because of the assumption that there will be more crime committed there, an assumption that, of course, becomes a self-fulfilling prophecy because you can't arrest someone for committing the crimes that you aren't watching. All of this has been amply documented, and I can provide sources if you wish.
The phenomenon on the SJW insult is an interesting one. Personally, I think that it has developed because there are certain people who have gone through their lives being able to make fact-less baseless groundless assumptions that are complete nonsense as represented in the quote above and have it go largely unchallenged. In the past decade or so, there has been more organization and more communication amongst progressive groups and this has had the effect that it has become more difficult for that kind of nonsense to pass without thoughtful rebuttal. Thus, the PC world was born. In other words, before the PC shift, I was able to get away with saying whatever false thing I wanted to say on the internet and now I can't. But, of course, for those who have accepted these sorts of massive distortions as the truth, it must look like the world has gone all PC.