I think this is similar to the fuss about ECN in the Linux kernel - in an ideal world, everyone would follow the spec; in the real world, the yum developers are making life difficult for people behind these proxies, over which they may well have no control. See also Postel's prescription, TCP window scaling and standards mode - in this case I think that these proxies are not going to go away, and it's churlish not to work with them.