It's not an American cultural norm. It's just English grammar.

Things which are written as compounds in other languages often have a space in English, and English does not distinguish between word classes so that Noun-Noun compounds and Adjective-Noun phrases are ambiguous.

But if you hear “white nationalism” five times, when you hear “white nationalist” you don't hear it compositionally as “(white) (nationalist)” but as “(white nation)alist” - someone who advocates for a white nation.

“White nationalism” is a word. It has a different meaning than Nazism in my experience, so calling for substitution would be wrong. For instance, a person could support a multi-party white nationalist pseudo-democracy like the US during slavery. But nazism is going to be a form of dictatorship.