It's not circular at all. Rephrased and generalized: "If more people end up in jail, because people are committing more crimes, it's not ____." Now add race: "

It's not circular at all. Rephrased and generalized: "If more people end up in jail, because people are committing more crimes, it's not ____." Now add race: "If more black people end up in jail, because blacks are committing more crimes, it's not discrimination towards blacks, but discrimination towards crimes". But yes, to your point, you can't judge solely based on how many are in jail, that they must be committing more crimes... because the justice system isn't perfect, and innocents do end up in jail. But again, to the other's points, it's entirely in the realm of possibility that black people are in fact committing more crimes, and hence there should be disproportionately more of them in jail. As to whether that's actually the case, is not easy to say.