It seems the paranoia of the US Government in the 40's knew no bounds.
It's a classic film that conveys the message that no one is insignificant, and friends and family are more important than money. It was, therefore, deemed anti capitalist. In the eyes of the US government, everyone was either capitalist or communist, so, therefore, the film was considered communist..
http://www.theregister.co.uk/2006/12...a_commie_memo/