Strong-arm tactics from MS regarding IMs ?

“Microsoft is forcing people to upgrade to newer versions of its instant messenger application and is shutting its doors to third-party IM products such as Trillian“   (via Royo)

At first glance, this seems like another one of Microsoft's underhanded marketing tactics. Closing MSN to third party IMs would force me (along with many others) to install MSN whether I want to or not (I happen not to).  As a big fan of XML, webservices, RSS, etc I just don't have the patience for these “My proprietary IM is better than yours” wars. But before I go off on a rant about MS shafting customers just to improve market share - does anyone have any additional information on why Microsoft is doing this?   What are the “security concerns” mentioned in the article?