Expect: 100-continue and proxies

2616 s8.2.3 first bullet of the requirements for proxies has a MUST 
requirement that a proxy forwards the Expect header to the next hop.

Apache httpd complies with this (whether by design or by accident, I'm 
not sure), though it makes little sense: the proxy is written to read 
and forward the entire request body without waiting for an interim 
response from the server.

Why is a MUST condition for the server to forward the 100-continue 
expectation appropriate, regardless of how a proxy handles the client's 
request body?

Regards, Joe