I'm not sure the Shawshank Redemption is the best example tbh. Maine is one of the whitest states in the US (95% of the population) so obviously its prison population is going to be much whiter (although not as white as you might expect...) than the prison population of most other states, and if anything would have been even whiter in 1949 than today. So just because the prison in The Shawshank Redemption was predominantly white, doesn't mean that that was the case everywhere else in the US.