Perhaps SES is a proxy for race. In any case, there is no good excuse for justice and safety to be dependent upon SES in a fair and just society.
They know which is which.
When you are analyzing the data you look for what variables are needed to get the best fit for the data and which ones are useless. For example, in this case:
1) Poor = more likely to convict.
2) Black = more likely to convict.
Now you do test #3, two variables, poor & black.
Which produces the best fit? If race is a proxy for SES then curves #1 and #3 will be equal and #2 will be worse. If SES is a proxy for race then #2 and #3 will be equal and #1 worse. If #3 is better than the others then both matter. When faced with two tests that produce equally good fits and one contains an extra variable you discard that curve, the extra variable is merely a proxy.
In practice
honest research into racial issues almost always ends up with poor being what counts, race only being a proxy. Dishonest research almost never does this test in the first place.