Until recently I had just taken it for granted that it was what we do, but after thinking about it, what's the rationale behind taking away the right to vote when someone is convicted?
Felon disenfranchisement tends to hurt the black population more because they're often more egregiously made felons through minor offenses (for example, possession of marijuana). This goes as far back as the Black Codes in the mid-1800s.
Bringing up Nazis is a little more then ironic in this context. Considering the Nazi's passed laws to remove citizenship and prevent voting for a large portion of their population.
On September 15, 1935, at a party rally in Nuremberg, the Nazis announced two new laws that changed who could be a German citizen. The Reich Citizenship Law required that all citizens have German “blood.” As a result, Jews and others lost their rights to citizenship, which not only stripped them of the right to vote but also made them stateless. This meant that they could not get a valid passport for travel between countries or acquire a visa to leave Germany.
Allowing everyone to vote is fundamental to democracy.