It's because bones are used in the processing of some cane sugar. I'm not vegan, this fact just randomly popped up when I looking at some dairy-free recipes which also happened to be vegan, so specified 'vegan sugar' on the ingredients list. I was sneering a bit at the recipe but Googled it anyway and was very surprised.
I'm assuming that most sugar sold in Europe comes from beets, which doesn't require the use of bones to process, but if you buy something that contains sugar as an ingredient it doesn't usually state the source of the sugar.
Did everyone know this already, or have I created a new 'I never knew X until MN told me' fact? 