I see what you're saying, but there's a certain nebulousness to the organization, and a reasonable suspicion that not everyone who claims membership is a credible threat (never mind that darn old Constitution that guarantees free association). The mere fact that we don't like an organization, or that certain parts of an organization are unpleasant, doesn't mean that we can make association with an organization unlawful (and certainly not a capital offense). That line becomes far too blurry far too quickly.