>but because it tends to produce human rights respecting outcomes
From the communist's point of view, they agree with this - communist movements until shortly after the second world war stressed non-involvement and the illegitimacy of "bourgeois democracy". The communists contend that the mixture of capitalism and democracy does not yield human-respecting outcomes, even if they may produce outcomes that correspond to the liberal conception of human rights.
If there is no space to disagree with liberalism - from its economics (capitalism) to its conception of human freedom (the rights system and the law of the state), where does that leave us? I think it results in dogmatism and ideological conformity.
Liberalism and capitalism should be able to withstand criticism and rhetoric without banning such criticism because of the symbols used to represent it. And it really is just criticism - these people are trying to get elected.
For a political movement, symbols are not just for those on the inside...
What if those people holding Nazi symbols want human-rights respecting outcomes that are very different from the ideology of the German Nazi party? If democracy is not just abstract and symbolic, you shouldn't dismiss a political party as undemocratic just because they use the wrong symbols, but look at the specific actions they take (the same holds for parties using the "right" symbols and paying lip service to democracy while trying to subvert it).
For example, the Indian state of Kerala is currently ruled by a government led by the Communist Party of India (Marxist), not in a one-party dictatorship, but as a coalition government of several parties. They have transitioned peacefully from government to opposition and back again multiple times over the decades. And yes, their party flag is red with a big hammer-and-sickle emblem. Does that last detail make them undemocratic?