In the explanation of setPropertyWriteHandler(), it says:

If no write handler is initialized for any given Property, implementations SHOULD implement default property update and notifying observers on change, based on the Thing Description.

I think that it should be rather:

If no write handler is initialized for any given writeable (or non readOnly) Property, implementations SHOULD implement default property update and notifying observers on change, based on the Thing Description.

Otherwise, it gives the impression that a Scripting API implementer should add a write handler for properties that are readOnly as well.