There’s nothing special about that phrase in particular (you just said it yourself).

The difference is between opinions (which are protected) and perceived threats (which are not).

Yeah I always view it as the rights of an individual end where anothers rights begin. Someone who reasonably has to fear for their health and safety do to your words/actions is having their rights trampled. It just gets dicey because people conflate hurt feelings with reasonable means.

If Terry says he really wishes someone put (insert politician name here) through a woodchipper. Terry is venting and people wouldn't without a reasonable doubt believe Terry was going to do that. Now if Terry rents a woodchipper and truck to pull it the next day and starts driving to the city/state where that politician lives... A jury of Terry's peers may suddenly believe they meant to do such themselves, especially since they didn't have any contracts to mulch trees or property they owned in that area.

Free speech ends when it encroaches another's freedom. When you start saying things that could lead to someone actually being endangered or subdued in response, that's no longer protected. It can get more complex, but that's the starting point.

Free speech doesn't mean you can't be countered with someone else's opinion though. Some people seem to think they can say things and then shout their freedom is being taken away when someone else disagrees. There isn't harm there except to their opinion, and that is free to shoot down.

Another way to look at it is a rule commonly found on online forums. Attacking ideas is supported, attacking a person is not.

The reasonable person standard is important here. For example, a reasonable person would not interpret a picture of some shells on the beach on Instagram to be an actual threat against a politician.

This is different from reasonable doubt which is a standard of proof required to reach a criminal conviction.

From the second link:

It is a higher standard of proof than the standard of balance of probabilities (US English: preponderance of the evidence) commonly used in civil cases.

Well, there was that one particular american president who got shot in the head. Then some century later, there was a second one. So I'd say there's two examples of precedent as to why it's something which evolved naturally, through the passage of time.