"While some might criticize Google for banning National Vanguard even though that publication's writings are protected speech under the First Amendment"
Clearly, they are not been banned, just excluded as people don't wish to read their neo-Nazi filth. National Vanguard can still, sadly do, publish the putrid mutterings but people are not obliged to accept them as a valid new source from Google.
Would anyone sensible accept Hitler's speeches as a valid news source? No, therefore we should make a critical distinction between the rantings of neo-Nazis and legitimate news sources. Dragging out the US Constitution to defend the proliferation of neo-Nazi ideas is as unworthy as it is bogus.