A node that is referenceable has an independent identity from its position in the workspace hierarchy (by virtue of its UUID): it maintains its identity regardless of where it is moved in the hierarchy.
Non-referenceable nodes, on the other hand, are intrinsically tied to their position in the hierarchy relative to their nearest referenceable ancestor. If a non-referenceable node is moved from its position it becomes, in effect, a different node.
Consequently, a referenceable node and its non-referenceable sub-nodes form a natural unit within the WS hierarchy. It is this unit that is respected during a save, update and merge.