1. They don't
2. Elon's maternal Grandpa, the Chiropractor, is originally from Minnesota. He's a dual citizen.
It used to be you could only be dual in the USA, yet now it's a free for all for whatever reason.
It also used to be that if you were a dual citizen, you relinquished your right to vote in the USA.
Elon has at least 3 citizenships... South Africa, USA, and Canada