You seem to misunderstand US law. The only comparative speech which is banned in the US is "fighting words," which must meet the much more strenuous requirement

You seem to misunderstand US law. The only comparative speech which is banned in the US is "fighting words," which must meet the much more strenuous requirement of "those that by their very utterance inflict injury or tend to incite an immediate breach of the peace." If the words themselves do not cause an imminent and immediate threat of violence they are protected.