I’m interested in general answers to this question. It seems like China and Russia in particular have strong protections against external influence.
What is stopping the US from doing something to filter out such attacks.
I’m interested in the legal, technical and geopolitical reasons, but also whether it’s even a good idea.
In the pre-industrial era, yeoman craftsmen (think Paul Revere) would hire a person to read them history and literature while they worked. In the early industrial era, factory workers imitated this tradition and also published/read dozens of leaflets we might call newspapers. Because this active intellectual life produced crazy ideas like "maybe let's do a union", it was quietly eliminated.
As time passes, we are increasingly conditioned to respond only to "expert" opinion (always conclusory, never persuasive), or emotionally potent oversimplifications intended to manipulate opinion in a particular direction.
When you condition a population to respond to propaganda, and only propaganda, you get predicable consequences.
The remedy is to encourage critical thought by socially rewarding activities like reading and reasoned dissent. Reasoned dissent is particularly unwelcome in communities of highly credentialed "intellectuals".
Could you imagine the Federalist / Anti-Federalist debate playing out today? It's unthinkable.