There are many possible reasons that the American justice system is much tougher on blacks (higher percentage of interactions with police relative to the amount of illegal activity – especially w/r/t drug offenses – higher percentage of arrests relative to the number of interactions with police, higher percentage of trials relative to arrests, higher percentage of convictions relative to trials, stiffer sentences relative to number of convictions for identical crimes) than on whites, which I’d speculate include but are not limited to: (1) blacks live in neighborhoods with higher crime and neighborhoods more hostile to the police, (2) the harsher treatment of blacks by the justice system is taken (perhaps subconsciously) by authorities to imply that they are deserving of this treatment ("more dangerous" or whatever) or perhaps just making those authorities used to the difference thereby perpetuating it, (3) blacks have more fragmented families than whites relative to income, partly because of these prison differences and because family instability persists across generations, (4) the black subculture (language, mannerisms, etc.) is seen as inferior by the dominant culture and blacks who interact with authorities are perceived as less intelligent or less articulate than they truly are because of these differences (5) blacks live in communities where crime and prison are more common and therefore less stigmatized, (6) schools in poor black communities are less effective, with different attitudes among administrators, teachers, and students than schools in other neighborhoods, and dropping out is less stigmatized, etc. etc.
Many of these differences are not conscious oppression, necessarily: it’s hard to fault a cop for being more on edge in a really dangerous neighborhood, for example, and that increased edginess is necessarily going to impact innocent people; biases related to the use of non-standard dialects (“ebonics”, etc.) are extremely difficult to overcome, and at a couple of points I’ve been shocked with my own bias after figuring out that someone I was talking to was smarter and more perceptive than I gave him credit for, simply based on dialect... and I try really hard to avoid such judgments.
Anyway, the point is, society is complicated, and simplifying big parts of it down to condescending offensive explanations, even if they sometimes have bits of truth to them here and there, is a good way to make people either pissed off at you or dismissive, without really advancing productive discussion.