You're not on a diet for not being a cannibal
I'll be preaching to the choir here, but I wanted to share these thoughts somewhere.
First of all, there's nothing wrong with being on a diet, obviously. However, a lot of meat eaters seem to think that vegans are on a diet. I've heard many times "oh, I forgot you can't eat dairy". No, I very much can, but I refuse to.
Meat eaters seem to believe that veganism is based on health I guess? So many of them don't realise (like I haven't for a long time years ago) that veganism is a moral stance, akin to disgust towards cannibalism.
Vegans can still be on a diet. If you have trouble digesting gluten, you're probably on a gluten-free or gluten-low diet. Or if you have diabetes, you probably avoid carb-dense foods.
This might come off as just a matter of petty linguistics, but I think it's so important for people outside the vegan community to know that veganism is 100% a matter of ethics, and I think this comparison to cannibalism is pretty effective.