Microsoft Requires DVD Drive But Not TV Tuner for Media Center

Matt Goyer responded on his blog to my question about Dell requiring a DVD drive to buy Windows Media Center:

Actually, it's, us, Microsoft who requires that OEMs configure their machines with DVD players.

I’m sure there are some licensing issues or what not going on, but I wonder why Microsoft didn’t require a TV tuner card? I have seen a lot of systems sold without a TV tuner card – what do you think the impact on sales is with or without a TV tuner card?