My understanding is those "usage rates" are uniform only if we look at "used in last year", but ignore "used in last week" metrics where they diverge (according to Bureau of Justice Statistics). SSC did a pretty good deep dive on race and the criminal justice system, that covers this and other things such as underreporting, below:

https://slatestarcodex.com/2014/11/25/race-and-justice-much-...