This is racism, pure and simple. It may not be intentional, but that doesn't change it. The frequency and types of crimes committed by powder and crack users are a function of their socio-economic backgrounds, not their drugs of choice, which are a co-incidental cultural preference which was convenient in the repression of lower social classes. The same thing happens now with white meth users.
Yes, they are different drugs. No, one is not considerably more addictive or dangerous than the other.
Plenty to read on the subject in the references here: http://en.wikipedia.org/wiki/Fair_Sentencing_Act
(Note my emphasis) You are begging the question here.
Consider the alternative hypothesis, that things went to shit and they switched to a cheaper form of cocaine. 'And', not 'because'. In fact, one can hypothesis that both changes were triggered by another factor that you have failed to mention.