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

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