<attributeGroup ref="..." use="prohibited"/> not in spec

I think that schema writers would expect the same control over derived
complexTypes whether the base type contributes attributes
* individually (with <attribute> element info items), or
* with a named attribute group (with <attributeGroup ref="groupName">).

So it seems a little asymmetrical that one is allowed to write <attribute
name="a" use="prohibited"> but one is not allowed to write <attributeGroup
ref="groupName" use="prohibited"> to mean: none of the attributes which are
part of that group should appear when this derived complexType is used.

Question:
Was this an oversight, or a conscious design choice by the working group?

Thank you,
Bob Schloss
IBM Thomas J. Watson Research Center