In 1.0, titles applied only to Works and Instances. Subject and creators applied only to Works. What we found was that for some (admittedly rare) cases, title, creator, and subject could apply at the Item level.
Complexity of constructed classes
Let’s say you want to restrict titles to Works, Instances, and Items, by imposing a domain on property bf:title. You can do that in a number of ways, including constructed or artificial classes. We decided that the benefit of doing this is not worth the cost of the added complexity this would impose.
As another example consider property hasEquivalent. A Work can be equivalent to another Work, an Instance to an Instance, or an Item to an Item. But a Work cannot be equivalent to an Instance, or an Instance to an Item, etc. In other words, the resource type of the subject (Work, Instance, or Item) must be the same as the resource type of the object. This would be a logical constraint to impose, if it could be imposed in a sensible manner. But it cannot, and so the range of hasEquivalent is unconstrained.