Why IE ignores "VARY" header and not compliant with RFC?

[msnews.microsoft.com]

Hi,

 

Here is part of HTTP RFC (http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html)

 

An HTTP/1.1 server SHOULD include a Vary header field with any cacheable response that is subject to server-driven negotiation. Doing so allows a cache to properly interpret future requests on that resource and informs the user agent about the presence of negotiation on that resource. 

IE completely refuses to honor this VARY header and fetches new page each time regardless of Cache-Control directive if it encounters VARY header in server response. Now, RFC compliant browser (like Mozilla) sees that header and make separate requests for separate VARY headers specified).
I have been told that it's something which Microsoft IE development team does not consider a bug and I would really like to know what is behind that decision.

L