[Serusers] Dubt on RFC 4484 and SIP Proxy Server behavior

[Francesco la Torre]

Hi all,
I've a doubt regarding a statement at the end of page 5 

"... Some important design decisions are associated with carrying
assertions in a SIP request.  If an assertion is carried by value, or
uses a MIME-based content indirection system, then proxy servers will be
unable to inspect the assertion themselves.  If the assertion were
referenced in a header, however, it might be possible for the proxy
to acquire and inspect the assertion itself.  There ..."

Why a proxy server will be unable to inspect the assertion themself ?
SER has functions by which it could be read message body, does this
behavior not respect any rfc ?
I'm missing something :-S

Thanks in advance,
Francesco la Torre