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And in other contexts, you can imagine supporting the right of Communists to speak and organize even if you find Communism abhorrent and misguided.

Note too the benefits of allowing/supporting the right of [insert disliked political group] to speak: 1) You find out who is in that group when they speak (as opposed to them remaining anonymous/hidden online) and potentially address them directly 2) You might be able to question them to find out what led them to that position to prevent more people from going in that direction 3) You can be aware of the message that they are trying to send and counter it with more speech

In contrast, doing something like banning support for the Communist party is likely to lead to 1) more sympathy for Communism as an oppressed outgroup; 2) a lack of understanding of how extensive support for Communism is and what might be causing it; 3) the use of "Communism" as something that people can use to denounce each other as part of petty/unrelated feuds.