Pre- diagnoses I knew approximately nothing about type 1 diabetes. Wouldn't have surprised me if I asked someone if it was because of their diet.
A minute of your time telling them, in simple terms, it's an autoimmune condition and your body decided to have a fight with your pancreas (and won), hence you can eat what you want because now instead of producing your own insulin, you have to have injections. If they ask about food beyond that bore them with sugar spikes, carb counting and GI. I'm yet to have someone not understand after explaining it - there's no need to be rude or be particularly offended by their initial ignorance!