Law is a funny beast. Lots of people do things which are illegal all the time and get away with it because you basically have to assert your right to be protected by law to sort of activate it. Like someone yelling at me that they are going to kill me while I am out in public is technically a form of assult. , I can call the authorities and get them to assist me to make sure they don't follow through and to get them to stay the hell away from me but chances are I am not going to seek restitution in court for something that small because I would have to press charges, seek and pay for legal council, everything would need to be processed to make sure the law is being properly handled at all points of the arrest and the punishment would likely be fairly trifling for all my troubles.

Private entities already basically have the imperitive to determine what is permissible on their platforms. Freedom of speech is not practiced under the auspices of substack. They are allowed to kick you out for whatever the heck they want (some exceptions applying) because they own that space. To remove posts as threats a judge would have to go through each individual one, source it, bring the original commenter into court and go through due process with every single user to check it against their local jurisdiction's laws for threats and the likely outcome would just be small fines and community service... Quite frankly the juice would not be worth the squeeze.

On the other hand we are absolutely allowed to have an opinion that substack letting Nazis spread hate speech on their platform under their watch is a moral failure on their part.