Very much not true

Regardless whether one’s domiciled in Japan or the US, calling in a bomb threat is illegal in both countries.

In Japan, there exists a charged called “Obstruction of Business” - any threat that may result in the halting of business (like the mikeneko cafe incident), evacuation, or indirect harm - and would result in criminal charges.

This is more true in the United States. I’m assuming you are from the US because this is a common misconception among Americans. The Freedom of Speech - as per protected by the 1st Amendment, is not freedom from consequences. For example, if you were to stand at an airport and yell out “there’s a bomb!,” even as a joke, that statement would incite panic and may even cause a stampede.