You realise that, prior to Windows 7, that was pretty much the situation anyway. Plenty of people watched DVDs on XP and Vista without ever being aware that the OS itself didn't ship with playback codecs in most cases. TBH most OEMs seem to like to bundle media playback stuff anyway (most of which is genuinely awful), so you've probably actually paid for codec inclusion twice when you buy a PC anyway.
LOL Andy. Caught playing Microsoft Apologist again I see. Hmmm... And the reason Microsoft started including the DVD decoder in Vista was???? Because they were just being nice guys? They didn't know any better? Somebody smoking crack?
Maybe it's because they wanted to provide a consistence DVD playback experience across Windows. I know that's hard for you to imagine but please try.
And on another note it looks like VLC is Microsoft's official answer to the problem they created based on this tweet from Building Windows 8 twitter account.