Depending on the size of your iTunes library, the XML file that needs to be loaded to AppleTV (along with all the album art) might be quite large. I'm guessing that's why they don't have all the libraries loaded automatically.
It also makes sense if you consider that you can stream from up to five computers - if my friend comes over with his MacBook, I might want to stream something from him, but I don't want to have all his music and movies pop up into my AppleTV library automatically and get displayed next to mine. That could be quite confusing.