MAFKKA on 11 November 2009
Their official reason was that VAC(Steam) and Matchmaking would be superior to PB and DS in regards to Cheaters, and piracy.
But now http://www.1up.com/do/newsStory?cId=3176882 cheats are already working, and all COD4 cheats are working aswell on MW2.
And as for piracy: http://www.rlslog.net/call-of-duty-modern-warfare-2-razor1911/ the game was pirated and up for download (360 version) 10 days before release.
