The AppleTV serves a sole purpose, to make the music and videos from your computer available on your TV. This simplicity is marked by the usage of the very simple Apple remote.
Adding in web surfing or any other normal computer functions would be a step in the wrong direction. Apple likes simple, so adding a complex feature such as web browsing would run counter to that mantra. Also, since browsing cannot be done with the included remote, Apple would have to either make a more complex remote, or allow you to connect a keyboard and mouse. Since mousing on your sofa is a bit difficult, Apple would have to create a keyboard with a built in mouse to allow for proper usage. As you can see, this is all unnecessarily complex for what is supposed to be a simple machine.