There's no reason why they have to become naturalized to live in the US and do 'volunteer' work for a non-profit organization. I know of many people from other countries that work and live in the US legally that aren't required to become US citizens. Similar arrangements are made for non-profit organizations to use volunteers from other countries.
I bet a more careful look at the US laws governing non-profit organizations would show that it is not necessary to give up your citizenship.
I know of several ex-JWs on JWD that are citizens of another country and work in the US legally and have for several years.