So, he deliberately chooses not to do that which would give him the right to vote.
Correct, that is his choice.
And just how is that discriminatory, or a failure on the part of the US?
It is not, nor did I say it was.
That's his choice, and he has continually made that same choice for decades.
Exactly, as I stated in my post.
Your father just doesn't value the right to vote enough to do what it takes to become a citizen. However, I don't see how holding dual citizenship would deny his national heritage. Becoming a US citizen does not destroy or revoke any previous citizenship.
Dual citizenship is kind of a grey, murky area. Technically, under US law, there is no such thing as dual citizenship as you are required to renounce your allegiances to any foreign flag when you naturalize. In reality, this is rarely enforced. As I stated in my original post, this is a choice that my father has made, knowing full well what the ramifications are. My point was that here is a man that has been a productive member of society, worked his entire life, always paid his fair share, always followed the law, and served in the US military. I don't think giving him a vote would be such a terrible thing.