no-transform & working group last call for -p1 et al

The no-transform directive forever has said that proxies MUST NOT touch
payload. 

Situation:

Suppose there is malware on a web site and a proxy resides between the
client and server. 

Questions:

 1. Why would the malware distributor NOT want to issue the no-transform
    directive?  After all, they don't want their malware removed.
 2. Why would a proxy honor the directive, knowing that there is malware?


My point: I wonder if the MUST is a bit too strong or whether a caveat
should be added around this.  (Maybe there is such a caveat and I've
just missed it?)

Eliot