> illegal
where?
On the internet, where project gutenberg is located. The argument put forward, that you repeated, implies just as much that someone could be prosecuted in a Chinese court for publishing a harsh (to the point of being illegal in China) criticism of China named freexinjiang.com, at least if the site included a Chinese version. And that, since it refers specifically to a Chinese province/area, would have at least some nexus to China, whereas Project Gutenberg has zero connection to Germany.
Arguing the connection by language (the fact that German-language books of note exist to be digitized does not imply a connection to Germany the country) or Gutenberg's biography (what does the nationality of Gutenberg the historical person have to do with anything? Is shakespeare.com subject to uk laws?) is insanity.
> I really didn't know [.us isn't commonly used]
I didn't realize how this might not be intuitive to people in other countries, so...
The U.S. mostly invented / was the testbed for the internet. It's standard in the U.S., and for almost any English-language website that's not seeking to be tied to a specific country of origin, to use .com .net or .org. I get that people in countries where English isn't official or where they're targeting the country's population as visitors will prefer .cc tlds either to indicate that their site targets that country's residents, or targets people who speak that country's primary language, but .us is not at all the norm in the USA; .com .net and .org generally serve that purpose, and .us is a distant fourth [1].
But, supposing .us were the primary domain used by U.S. organizations, even if Project Gutenberg didn't use .org but instead used a generic TLD, it would still have zero to do with Germany.
[1] It's actually 6th, behind .xyz and .info, but those two probably have a higher percentage of spammy junk websites and email domains, so .us might be fourth in domains that aren't junk ecommerce advertising sites.