The fact of spreading some ideas can lead to a banning so yes, KKK shall've been banned, as being banned they'd have spread their offensive and liberty-threatening ideas, now maybe they are too known and banning em will create just somewhat a martir,so they can go saying shit they are opressed by a niggers-lover government or such, just taking action against some kind of people is hard.
BTW here in spain the political faction of ETA( the terrorist group in here pro-independentist) has been banned 4 showing support to those terrorist actions, and that doesn't attempt against their freedom of speech, as far as their attitude attempted to many other people (mostly politicians) freedom of life(harrased and threatened by those terrorists when they crossed in that political group path)