corn and rice

Question

Dear Rabbi, It seems to me that corn-on-the-cob boiled by a non-Jew and rice cooked by a non-Jew should be equally prohibited or permitted. Also, I do not understand why it's okay for a food like chocolate or chewing gum to be produced by a non-Jew and be considered kosher(even without supervision), but it's not okay for a Jew to eat rice in a restaurant. I fail to see the reason why food that is prepared by non-Jews but packaged is kosher. If a restaurant "packaged" the rice for sushi, would it then be okay? There must be a way to find a leniency to permit eating sushi at any restaurant. Thank you for your time, Rabbi.