> what is a "basic right" supposed to be, anyway? I missed that day in philosophy class.

Fundamental rights that are inherent to all humans by birth and cannot be justly alienated. It's a major philosophical topic you've probably seen reflected in the universal declaration of human rights, american declaration of independence, french revolutionary declaration of the rights of man etc .

There's a terrible and anglocentric wikipedia page devoted to this topic: https://en.wikipedia.org/wiki/Natural_and_legal_rights

It's not even due to law; it's due to the founding nature of the Republic. Arguably not even a constitutional amendment could permit restrictions on Americans' interstate travel.

> Arguably not even a constitutional amendment could permit restrictions on Americans' interstate travel.

You might think it would be arguable but John Gilmore discovered you can't even argue it -- the law is secret and cannot be discussed in open court, thus he had no standing to sue over it: https://papersplease.org/gilmore/ https://en.wikipedia.org/wiki/Gilmore_v._Gonzales

The wikipedia entry is slightly misleading on one point: the ninth circuit said he could travel without ID (this case came about because I had been flying without ID and discussed it with John) -- his suit was that he had no choice due to a law he was not permitted to read (yet ignorance of that law is no defense!) and he argued that that was a violation of free speech + free assembly. Without being able to discuss the law in open court, the justification for the decision was a fig leaf.