Currently you may only claim citizenship by descent if you are under 18
An adult can claim 'derivative citizenship' through a US citizen parent. It's more complicated than birthright or descent and includes a length of US residency requirement for the parent. It's also more complicated if the US citizen is the father AND the parents never married.
"Applicants 18 years old and over, born outside the United States, may claim U.S. citizenship from a parent who at the time of the applicant’s birth was a United States citizen. Once the citizenship claim is established, the applicant qualifies for a first-time U.S. passport."
IF the argument is that non-resident legal aliens are not subject to the jurisdiction of the US, what would stop non-resident legal aliens from claiming the equivalent of “diplomatic immunity” for day to day things like car accidents, speeding fines etc?
Exactly. They're going to have to twist themselves into knots with pointing to one small part of the law while still ignoring the ramifications or negating the effects of their specific interpretation. "No, they aren't subject to the jurisdiction. Unless they violate the law (or we otherwise want them to be)". Contradictory, but what else is new?
The 'new interpretation' is that you are a 'natural born citizen' IF your parent(s) are US citizens (birth or naturalized) regardless of your place of birth OR your parent(s) are Lawful Permanent Residents (ie 'green card') AND you are born in the US. Children born to people who are here on temporary visas (visitor, work, etc) or here illegally are not. From what I can tell, that's more along the lines of what most European countries do. I wasn't aware that birthright citizenship was so "Americas-centric", but it seems to be if you look at a map of who does what.