They do clarify their stance on this in their terms and conditions. Basically when they say unlimited it just means that they don't have an arbitrary number limit but instead it is restricted to pretty much anything reasonable which makes everything easier. They do say that they are not an online backup service and for me that is fine, that's not what they are there for.
ISPs do a similar thing when they offer "unlimited" downloads. For most intents and purposes it is unlimited simply because it is more than will ever be used.