I didn't see this answered in the links: apparently then, some states allow felons to vote, and some do not. Does this apply to federal elections as well as state elections? In other words, some people are allowed by their state to vote for a president, whereas some, by dint of their geography and record are not?

If this is the case, it is surely unjust.

Prisoners cannot vote in UK elections either, whilst incarcerated. Equally, in a ruling no doubt to be cherished by HoreTore, neither could the aristocracy (until recently, when most hereditary peers were barred from the House of Lords).