Australia

Because there are limits to freedom of expression when it infringes on other people's rights.

For example, if I walk up to someone and tell them I'll kill them if they don't give me all their money, that's outside the limits of free expression - it's a threat of violence, and hence that makes it a robbery. That is a reasonable limit of freedom of expression in a democratic society.

The same applies if, given mutually shared background and context, the threat is only implied. For example, if I walked into a bank and gave the teller a bag and a note saying "You'd better put all the money into the bag right now!", that is still robbery even though the there is no explicitly written threat, because it is implied from the context. The message sender (me in that example) and receiver (the teller) know how it will be interpreted even though the threat is left unsaid. Even if that particular bank robber has never hurt anyone, they rely on actual force used by past bank robbers to reinforce their message. Criminalising such robberies that rely on implicit threats is still a reasonable limit of freedom of expression in a democratic society.

Sometimes, no words are required at all; there are situations where a combination of clothing and actions / gestures also send a threatening message that both the message sender and the message receiver know are threatening. Dressing up in neo-nazi garb and throwing Nazi salutes is equivalent to shouting "If you aren't white, this place is not safe for you now". The people sending that message know that is the message - that is, in fact, why they choose to do it. The people receiving the message also know that. And the message is reinforced by occasional actual violence by neo-Nazis (even if not everyone sending the message actually has been violent). The only real difference from the bank robber making the implicit threat is that the threat is implied by actions instead of words. Criminalising symbols and gestures that send an implicit threat to people is a reasonable limit of freedom of expression in a democratic society (less so if the gesture is only used ironically to call someone a Nazi, but given the rise of actual neo-Nazis I think the law is reasonable, and there are plenty of other ways to criticise authoritarian politicians that should not be illegalised).