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