Because of the relation child -> parent, a content into a hierarchical reference table tree could not be deleted if it has children because its referenced by its children.
We need to delete all children before delting the parent.

Moreover, if the relation between the parent ant its chilren is a invert relationship (for exemple in thesaurus plugin) it was impossible to delete nor the children nor the parent.

For hierarchical reference table, the deletion button has to be overriden to alllow deletion if the content is ONLY referenced by its parent or by its children.