This mostly comes down to a battle between 2x platforms: Google vs. Microsoft. I consider myself a pretty avid Microsoft supporter, but if you look at the facts, I kind of think that Microsoft started this fight by:
1) Buying Skype and pitting Skype against Talk.
2) Their Scroogled campaign that pitted Outlook against Gmail
3) Connecting Outlook.com to the Talk API when Google would have preferred that Microsoft federate skype/outlook/hotmail/live/passport via XMPP.
It's that third point surrounding XMPP federation that this all comes down to. When Microsoft decided to not federate via XMPP with the Outlook/Skype consumer products they were saying that they only wanted to establish 1-way communication with Google's platform. There is no doubt that this pissed Google off because Microsoft is trying to take away their market share while also taking advantage of their services and open architecture. Google's offered up XMPP for many years and Microsoft never connected until they had a mail product that was capable of trading market share (in one direction).
Microsoft is clearly not against XMPP because they do support XMPP in their commercial IM product, Lync (which I'm a regular user of and competent in supporting/deploying). I've considered many scenarios but can't figure out why Microsoft wouldn't want to enable XMPP for its consumer products as a way of communicating with Google Talk contacts other than to discourage interoperability with their consumer products; e.g. keep everyone on Skype.
I know that some might argue that Microsoft connected to Google the way they did so that it could pull over all of your Google Contacts and already authorized XMPP invites, but in my opinion they could have just showed you a list of all your current Google Talk XMPP contacts and asked you to place check marks next to any that you wanted to invite to your Microsoft Account contact list. With all that said, maybe its as simple as that someone in the right position at Microsoft failing to comprehend the scenario.