I believe they’re referring to Chikan which is estimated to have happened to 2/3 of women who ride the train in Japan and the act violates several Japanese laws.
It is when the harassment is of sexual nature.
Actually all harassment is a crime in the US. Just insulting people and saying lewd things isn’t harassment, though.
I don't think this is true. There are 50 states, each with different laws and definitions of acts and crimes. I am not aware of any federal anti-harassment laws except for those governing workplaces, and even those do not consider all harassment to be illegal.