Because its the only use case that will make sense at the beginning. These videos look interesting but they are staged, no matter how casual de look is.

As of this moment, Google Glass is just a glorified wireless webcam. It might turn into something more than that in the future, no doubt, and I'm hopeful like you are that it will, but for the first generations what you are going to see is a lot of people using them in extreme sports to film POV activities and sharing them on Youtube. Nothing more.

Even if it's not only voice controlled theres no discreet-email-checking use case in social situations. Have you ever tried to talk to a person who is constantly looking away, even if his head still points to you?