Imprisonment except for life imprisonment is limited in time. It is based and justified on the purposes of criminal punishment. So limiting their rights for the limited time of their punishment is justified and necessary, but not afterwards. Also with capital punishment there is a reason why developed countries have outlawed it.

Punishment in a state of law typically has these purposes: deterrence, incapacitation, rehabilitation, retribution, and restitution

Deterrence comes from the threat of imprisonment or in smaller cases, fines, social work etc.

Incapacitation is given through prison sentences. There is cases where the person is deemed to dangerous to be left out afterwards, so some countries have the institution of preventive detention. It is distinctly different from imprisonment though, because it should not serve as continued punishment. There can be non detentive incapacitations necessary. E.g. sbd. who has molested children would also be barred from working with children after he served his sentence.

Rehabilitation is often negelected in the US and other countries. If the person is to be released after their sentence, the sentence should prepare them from being able to become a law abiding member of society. Taking away their rights to vote and other measures are keeping them out of society, and contradict rehabilitation.

Retribution is the prison sentence. For it to be just, the person has served its retribution with the sentence.

Restitution has to be decided by the court, for how it is possible to compensate the victims. But the victims are not compensated by a permanent discrimination against the perpetrator.