[Radim]

Isn't that the point though? Europe has experienced first hand not only "national socialism" (nazism), but also another type of socialism — communism. Still legal though, and going strong in Europe.

It's a grim comparison, but communism has actually produced a higher body count that nazism and fascism combined.

That US had traditionally granted free speech to all kinds of extremists, no matter the -ism, was truly remarkable and unique. Definitely a cause for wonder (even jealousy like OP says).

And the way they found to circumvent that pesky constitutional constraint by having mega-corp-in-bed-with-gvt monopolies do the dirty work, without breaking the letter of the law, is remarkable again. An elegant "hack".