The US dropped its blanket ban on anarchists and communists well over 20 years ago. I personally know people in both camps who have visited the UK and one who has permanent US residency.

I'd argue that while the US has many problems when it comes to its treatment of the left, things have moved in the opposite direction of what you indicate. E.g. the far rights attack on Obama coupled with generations now growing up who never experienced the cold war combined to make words like "socialism" far less scary.