If you cancel bad ideas instead of speaking vociferously against them, the bad ideas spread uncontested (just more slowly). Furthermore, it sends the implication that the bad ideas are in fact correct since otherwise one would just provide the correction. And if there are a mix of good ideas and bad ideas, the good ideas get thrown out with the bad ones, so we can't benefit from those. All in all, a bad idea.

And actual liberals--not the illiberal leftists who have been eager to cancel the right, who have been eager to erode a culture of dialog with "freedom of speech doesn't mean freedom from consequences" meaning doxxing people, getting them fired, etc., if they can--have embraced this point for a very long time.

As always, however, active calls for violence needn't be tolerated because they're not just ideas. The point of contesting ideas is to come up with the best ones, not for might to make right. And yes, there are some gray areas (e.g. what does "from the river to the sea" entail), but those can mostly be resolved by observing what people actually do. If people are in fact going out and invading the U.S. Capitol, "fight like hell" was, in practice, an imminent call to violence. Likewise, if people hear "from the river to the sea" and go out to attack Jews or bomb buses in Tel Aviv or something, then in practice it was an imminent call to violence. Otherwise, people can have their aspirations and other people can have other aspirations, and when they're radically mutually incompatible, one can seek solutions by dialog.