The basic reason I think it's worthwhile to push back on the "America has always been fascist" talking point is that it creates the impression of a sort of acausal, inevitable present. But America's current fascism has been the culmination of work by a surprisingly small number of groups and people, with plans and goals and means, which often enough we can identify. And that means we can understand it and fight it from a position of understanding.
America's anticorruption laws had to be totally fucked to get here, and they were, on purpose. America's election laws had to be totally fucked to get here, and they were, on purpose. Etc etc. The basic premise of pluralistic democracy embodied by a post Civil Rights era US has been under attack basically since it was born, and if America was always just like this the fascists wouldn't have needed to do all that.
That hardly redeems the many failures and inequities of the US as the height of its democratic health, but we can still sensibly distinguish one thing from another