Object setup: deleting object type doesn't alter fieldmaps

It looks like I can remove the object from the object setup tab even if I have a fieldmap that uses that object.

Either the fieldmap needs to be removed when the object is removed the object setup tab, or that object's checkbox should be disabled since a fieldmap is using it. In the latter case, the user would be expected to remove the fieldmap first before unchecking the object's box.