I do feel like that gives an incentive to get people of the opposite party into prison to influence the election.

1 vote on its own doesn't matter, however, the collective vote of undesirables in a country with the highest incarceration rate on earth could really fuck shit up for the elites who seek to control the population.

What is the rationale of not letting felons vote? I'm guessing a felon is something significant like murder or aggravated assault?

Most people would rather vilify than forgive.

Stripping felons of the right to vote was/is a part of Jim Crow, wherein blackness was systematically criminalized, usually through forced poverty and then a criminalization of poverty (e.g. petty theft for survival). Similar to a poll tax, the goal was to prevent black people from having a political voice, including but not limited to electoral. This is why these laws are mostly in former slave states. They were a reaction to liberation. These anti-black policies also applied to anyone else that would be systematically marginalized, serving as a reusable tool for the ruling class. Make poverty itself a deep pit of disenfranchisement and all you need to do is make your targeted group poor enough. Keeping the poor and precarious from organizing politically is also a goal unto itself for the ruling class, though we shouldn't get overly invested in the idea that voting would ever be enough to actually properly contradict the ruling class itself.

The criminal "justice" system is not about reform, certainly not in the US. Every aspect of it makes it harder to reintegrate into society afterwards, usually with your record following you well into your life after leaving the prison. Getting a job, finding housing, applying for benefits, all of these will be seriously hampered by being convicted of a crime and serving time. Instead, the criminal system is designed, again, to marginalize. Take the people that are a threat to the perceived interests of business owners and isolate and harm them, also attempting to create the appearance of a deterrent so that others don't want to threaten private property interests. This impetus poisons the entire system even when it deals with crimes that are not directly crimes of poverty or capitalist alienation (though the societies and pain constructed by the ruling class are certainly their fault).

Please note, however, that the fact that so many people are disenfranchised already shows us that the ruling class isn't going to let folks vote them out or otherwise engage in the political policies necessary to address injustice. They won't let us solve the climate crisis or systemic unemployment or treating housing as an investment. The overt disenfranchisement is a blatant example of how they tip the scales in their favor, but it is far from the only one; most forms of disenfranchisement are so deeply ingrained that few people notice them as such. Poor or biased schooling so that the public will accept propaganda narratives. The maintenance of an economic underclass stripped of rights (such as undocumented immigrants). A requirement to work so many hours that you cannot rapidly gain political consciousness. A media apparatus wholly owned by the oppressor class and obediently taking orders from it on what to focus on, which reporters to hire and fire. The elimination of public squares and meeting places by which to organize. The cooption of academia through a variety of means, ensuring that their work suits the goals of the ruling class or is at least stripped of its capacity to organize against them. The limiting of the concept of political action to voting and going to cop-sanctioned protests. Etc etc.

The way out of this is to organize directly with one another, to use our organizations to (further) identify the material root causes of injustice, and to work with more than just the tools offered to us by those who already have power.

Given how little one vote matters, we have a much more serious problem here: why should any individual vote?

For any one person, the chance that even one election in their lifetime will have its outcome altered by their vote is vanishingly small.

Therefore, in terms of practical effect, each individual always faces this awareness: that whether and how they vote is purely symbolic in its effect

There are two tricky parts that come with allowing prisoners to vote that must be considered. Not hard stops, but just additional dynamics that will be in place.

1. Prisoners have little to no autonomy, and can therefore be easily coerced into voting a certain way. If the warden/prison staff lean conservative and they hear that a certain prisoner voted liberal, that prisoner is vulnerable to reprisal. There would need to be an additional entity present in prisons to enforce privacy of voting results. But how do we guarantee that this government entity won't just collude with the other government entity running the prison?

2. There may be problems in terms of where these votes are counted for. One way to protect the anonymity of prison votes is to pool them among the district that houses the prison. But do we let the prisoners vote for local candidates/laws when they are not locals? In many cases, prisons are located in very small towns and may therefore significantly skew local elections if they participate in them. So does everyone get an absentee ballot for their place of origin instead? Even if the duration of their sentence means they are likely never to go back there? Or do prisoners only get to vote on items/candidates at the federal level?