The protagonists (literally Protagonist in Snow Crash) got to choose because they're connected, wealthy individuals with relevant skills. It's a very privileged viewpoint.

Nell's folks did not get to choose. The implicit dystopia is that of Nell's folks, who don't have skills, don't have connections, and have no way of getting out of their shitty situation.

The flip side of flexible nationality is if you're unwanted. Can nations choose to eject natural-born citizens who will incur too much medical cost during their lives? Or people who don't fit the nation's norms of sexuality, gender identity, religion, whatever? Can you suddenly find yourself ejected from your nation because an algorithm detects something in your profile?

Nations can and have done all those things.

Possibly an argument can be made that forced nationality based on geography tends to make those things less likely than they are in Stephenson’s fictional world.