There's a lot more to american society than standing in a voting booth. I agree it's a steep price, but there's no need to exaggerate.
Kids have no voting rights either. Or legal aliens. Or visiting diplomats. Etc.
If you were genuinely curious, "what's your opinion on voting rights for convicted criminals?" would be a better way to ask.
Asking nicely makes it more likely that you will get one.
People are much more likely to answer a question when it seems like you're interested in a genuine answer, than when it seems like you've already passed judgment and the question is a mere formality.
They were not asking in a pleasant manner, but they were also not being outright rude. Not every interaction has to be sunshine and lollipops.
Passive-agressive outrage at the apparent lack of concern for the disenfranchisement of a fairly large population group seems... ok in the grand scheme. No name calling, no yelling, and no 'please sir, will you answer me this' is required.
This advice parallels one of the Hacker News guidelines: "avoid gratuitous negativity". Asking questions in a gratuitously negative way doesn't usually make for good conversation.