Most people already own an HDMI cable or they are just going to use the same HDMI cable they were already using with their current Apple TV.
Most people here do, sure. But you'd be surprised how many first time buyers of the aTV come into a retail store, don't have an HDMI cable and then bitch about having to either drop $29 on the cable at the store, or bitch about going to Walmart to grab one for $5. It would be nice if they just included the cable, as they include a cable with every other product they ship. Especially one that's so crucial to the device.