White or brown sugar is made from sugar cane, or from sugar beets. Both of those are vegetables. Since sugar doesn't come from an animal, why do some brands of sugar bother mentioning on the package that they are "vegan"?
Apparently, the reason manufacturers label sugar as vegan is due to the way it's refined. If they use bone char (charred cow bones) to process sugar, then they won't label it as vegan.
It isn't a bleach bath that takes sugar from coarse and brown to fine and white. Most raw sugar is actually refined with a bone char. To make bone char, imported cow bones are incinerated and reduced to activated carbon, which can pick up color impurities in sugar [emphasis added]. White sugar, as well as brown sugar and confectioner's sugar derived from white sugar, comes into direct contact with the bone carbon throughout the filtration process. The sugar that ends up on grocery store shelves, however, rarely contains bone particles, which Zamojcin says makes it OK for some vegetarians to eat. But strict vegetarians and vegans still say no to refined, white sugar because animals were used in processing.
Words to look out for: There's usually one ingredient listed on a bag of sugar: "sugar." So to know for sure if your sugar is processed with a bone char you'll have to ask the manufacturer. Some sugar companies are explicit with their terminology, but others will simply use the term "natural carbon."
Alternatives: Raw cane sugar, beet sugar, and organic sugar are never processed with a bone char. Some white sugar manufacturers, including C&H Sugar Company and Imperial Sugar, have also opted out of the process and refine their sugar without animal byproducts.
I am not aware of any sugar that is not vegan and vegetarian. The closest exception is honey, which is obviously honey not sugar, and regarded as not vegan.
Likely you are just noticing that some products think they sell more by calling attention to the fact that they are "vegan", which makes it easier for vegans to buy and maybe some shoppers think things that say vegan are better; and other products do not think that.