that was the person you replied to's point, people threaten others every day, yet these charges would never be brought against them.
Because threats made by a random driver, often through a closed window, against another random driver in traffic are widely believed not to be serious. If the threatening driver proceeds to follow the threatened around, it does become criminal.
Besides prove it is a threat and not just you being over sensitive.
That what the courts are for: to determine whether a "reasonable" person would interpret the behavior as a legitimate threat. eg, actors on stage regularly shout "Fire" in crowded theaters, and you are probably clever enough to shout fire, as a member of the audience, in a way that people realize you're "joking."
That was one of the big changes after 9/11: it stopped being possible to "joke" under certain circumstances. Joking about carrying a (picture of a) gun through security gets your "gun" confiscated. Using the phrase "another 9/11" "another Columbine" (or, at VA Tech, "another 4/16") on the internet gets you a visit from NSA or the local police. There used to be procedures for distinguishing real bomb threats from idle or frivolous threats, but they're pretty hard to apply to 140 characters.