I'm with you. This seems like an odd move, given that most likely only a minority of their users used XP for some time. Why haven't they been warning Windows Vista, 7 and 8 users to use Bitlocker for years (not to mention Mac OS X and Linux users)? Perhaps I just missed the warnings, but I am really puzzled.
I suppose the developers could have just found a massive vulnerability (perhaps they're doing their own audit in advance of the public one?).