Diabetic foods aren't recommended by most specialists, so I think many companies are withdrawing or changing their products. Some have kept the product the same, but changed the name to 'sugar free' rather than 'diabetic', but others have just removed the products like it seems Thorntons have.
I think the assistant may have been referring to that advice (that diabetic products aren't necessary) but maybe phrased it badly. Obviously, we all have to judge what suits us and control our portions and the timings of treats like that, but I guess the assistant might not have understood the intricacies of diabetes and was just repeating what she was told by the company as to the reason.