Firstly, why do you actually believe this? Look no farther than the first comment made here that claims the entire left is an extension of communism. The right likes to raise the specter of communism all the time as a straw man argument to denounce all progressive thought. This isn't reality any more than it would be to claim the entire right is an extension of white supremacist thinking. Such binary thinking isn't actually useful nor intellectually honest. 

And why aren't communists stigmatized just as much as confederates? Maybe because I can't think of a single violent action of anyone claiming to be a communist happening in America. Meanwhile, confederates literally once started the Civil War, the most violent and pervasive embrace of domestic terrorism we have ever experienced. And their continued influence for a century latter suppressed, intimidated, and killed black people. And even today, their hate, violence, and terrorism is on display. There is no comparison.