SHHHHH! Why'd you go and rat me out! Actually it has less to do with ping, and more about the way the Azure service works in conjunction with the new Live versions. By default, Live will utilize YOUR unused bandwidth or part of your used bandwidth to assist others in your lobby and match. Basically, it's like taking the collective bandwidth resources available (cloud type service), and distributing them across the collective. So yes, if someone has a horrible connection and has like a normal 1200 ping, your own resources for matchmaking will be cut by like 25% (just throwin out numbers) and given to the bad connection people.
End result either way, is that this is not a very good way to do matchmaking. Your own resources should never be tapped to assist others with their bandwidth issues. If anything, matchmaking should be limited and grouped. meaning, that people with 72-200 ping should be grouped, 201-500 and so on. No one should have to suffer performance wise because others can't reliably provide their own bandwidth.