Nothing prohibiting advocacy of nazis, from what I can tell, it's for affiliation with nazis. And its an entry/approval requirement, and there is a big difference though between entry/approval requirements and ongoing obligations. The government can deny entry/approval for a myriad of reasons related to unfavorable speech, but they can't infringe on the legal speech of a green card holder.

Regardless, the prohibition for entry/approval is against people who were associated with the nazi party or nazi-allied parties between 1933 and 1945, which is basically obsolete already. Anybody for whom that prohibition applies would be 98+ years old now.