<IANAL> The wording of the second clause of the 14th Amendment implies that states are permitted to disenfranchise their citizens in cases of rebellion or crime, as it requires those states to adjust their allotment of representatives when they do.

Also, the Constitution doesn't explicitly state that citizens shall maintain or regain voting rights after having served their time, nor does it forbid disenfranchisement of voting rights, so the power to do so devolves to the states. </IANAL>

The disenfranchisement is seen as part of the sentence. Traditionally, the crimes deserving of this were very serious such as murder, rape, felony robbery, etc. The people committing these crimes were considered as having violated the social contract in such a manner that society could no longer trust them.

Unfortunately, many less serious offences have been classified as more serious in order to be "hard on crime" leading to many generally trustworthy citizens losing rights.