qualification of components of an all group

>  the components of a group whose <nt def="nt-compositor">compositor</nt> is (implicitly) <pt>all</pt> may not be qualified,
> and therefore call for exactly one appearance of the element they identify.

What's the justification for this restriction?  * and & don't interact
right in SGML, but I don't see why this can't be fixed in XML Schemas.

James