Qn about nested encryption

With ref to the proposal "XML encryption syntax and processing" v 1.0, dated 2000/12/15, by Dillaway et al, I hav the following question.
Section 2.5 states that "..it is not valid to nest these objects, i.e., an Encrypted Data may not be a child of an Encrypted Data."
I dont understand the reason behind this constraint.
Consider the case where a document is encrypted for multiple recipients.  It is reasonable requirement that recipient A is authorized to access an element X and all its descendents, while recipient B may is authorized to access the same element X less some of its descendents, say element Y.
A simple way to solve this is to first encrypt element Y with key K1, then encrypt element X with key K2.  A has access to K1 and K2 and must decrypt elem X and then Y.

I think, the following memo:
http://lists.w3.org/Archives/Public/xml-encryption/2000Oct/att-0011/01-myproof-xml-encryption-position.html
also refers to the same issue.

regards
SSH