I don't think it's that offensive (coming from someone who keeps Kosher himself.) You should not feel guilty at all if you don't want to keep Kosher. Not everyone who travels to Israel is Jewish. It's not like just because it's an Israeli airline, you have to be an observant Jew. Even people who observe the laws of Kashrut strictly don't grimace at the sight of another eating a cheeseburger. In fact, I think if you got looks from eating a cheeseburger, you shouldn't feel guilty, but the person giving you the looks is wrong. Matters of faith are a personal choice, and that included religious dietary requirements. Anyone who forces them down on another is wrong and ignorant.