Miss P, that's not true at all.
I'm diabetic. I eat all my favourite foods - they're all healthy but even if they weren't I could still eat them. I eat chocolate, that's no problem either. I have Type 1 diabetes - That's treated with insulin injections, not banning certain foods!
Even type 2 diabetics can eat what they like, as long as it's within an overall healthy diet, and some things are just for occational treats. The diet reccomended for type 2 diabetics (the ones you're thinking of, please be more specific!) is just what is reccomended for everyone anyway. It's just that other people think they can get away with eating whatever they like. Type 1 diabetes (which I have) is nothing to do with what someone ate, and I can eat the same as anyone. It's caused by an autoimmune reaction, not by bad diet. I don't limit sugar, or carbs, that's not how it works for type 1 diabetics. I just count how many grams of carb are in my meal, whatever I choose, and that determines my dose of insulin for that meal... your pancreas does the same thing for you, but you're not aware of it. My pancreas doesn't do it so I have to do it manually.
I however only drink diet soft drinks not the regular kind, as there is so much simple sugar in the regular kind that it would require me to inject quite a lot of insulin in order for it not to make me very, very, ill. Diet drinks don't make me fat. I'm at the low end of the healthy weight range, as are most of my diabetic friends who also stick to diet drinks.
Diet drinks don't cause weight gain themselves, but people with weight problems often make the mistake of thinking that having a diet drink will allow them to eat more and not gain weight. Also people who diet constantly often have a slow metabolism from yo-yo dieting, and they also tend to drink diet drinks as part of their diet. It's not the drinks, it's the overall lifestyle that needs a change.
I eat healthy, drink water, exercise, and take great care of myself... including occationally treating myself to whatever treat-foods I like, and managing them sensibly.
BTW, If anyone wants further information about the different types of diabetes, how they differ, and ways they are treated, just ask me. I'm happy to explain.