Who are we to tell them otherwise?
I also don't think that the "you're allowed to show up and not check a box, but you've still gotta show up" argument passes muster. That's very clearly an attempt to coerce speech out of someone which is also a 1st amendment violation.
You can be an absolutist if you like but in the real world the U.S. First Amendment has had limits since the beginning. Which is not to say that free speech doesn't need to be protected! It's just to say that the government has a legitimate interest in promoting its own democratic nature, so a mandatory voting system is possibly legal.
Don't forget that civil disobedience is an important form of speech. Think of not voting in this situation as a speech tax. It's free as in speech, not free as in beer. :P
Could you clarify? It's somewhat shocking to hear the Vote.org representative say voting is not a right in the U.S.