In Star Wars: Imperial Assault can all rebels attack multiple times?
"Hero" is a very special term in this game.  Only the main characters that you start the game with are heroes. You can pick up other, non-hero characters as you play through the game. Example: han solo.  All imperial characters are also non-heroes.

Heroes can attack multiple times. Non-heroes cannot (unless another rule overrides this). That means that imperial characters and non-hero rebel characters cannot (normally) attack twice per turn.  You are right in that it's functionally almost as though they have the assault ability. However, they don't have assault on their cards, so they don't have the ability. Attacking twice is simply a feature of being a hero.

This distinction could potentially be important later. For example, imagine if a mission had a rule that the assault ability doesn't work during that mission. Or that no abilities work during that mission. In such cases, the heroes would still be able to attack twice, but other characters with assault would lose the benefit of assault