Hi David,
It is SAP standard behaviour. Generally the requirements would get passed from the Sales order based on the Schedule line category. Here in your case you are tyring to create Delivery W/O reference to an order - That means the understanding is that you havre the necessary stock in place and that you are trying to create a delivery to ship it.
Please note that even for a delivery W/O ref - by standard Availability Chk happens.
Please let me know for any further clarification.
Regards