SAP allows removal of UDFs which are already assigned!

Hi,

I am using B1 9.1.

I added a new UDF in Item Master, set the Validation Type to Valid Values, and defined the Valid Values.

I opened the Item Master for some existing items, and assigned a value to this UDF.

After that, I tried to remove the UDF from the UDF Management window.

SAP just gave a warning that the process is irreversible, but still allowed the UDF to be removed even though it was in use in a few items.

Why is this behaviour and how can I prevent removal of UDFs which are in use?

Thanks.