I think the crux of the issue is that the elections are held by each state, and each state can't enumerate its own eligible voters due to the lack of a 100% com

I think the crux of the issue is that the elections are held by each state, and each state can't enumerate its own eligible voters due to the lack of a 100% comprehensive ID system. Think of European Parliament elections, but in this case there is no concept of "home country". However, I certainly do agree it's not ideal.