Wrong. If you have your songs synced to the ATV it plays without iTunes being open. So its playing off the ATV not being streamed as audio like on the AX.
Two things:
1. As I said before... nothing plays ON the AppleTV. Yes, you can play content FROM the AppleTV to an external device like a TV or a stereo, but nothing plays "ON" it. Maybe I'm just being unnecessarily clever... but believe it or not there are some fencesitters who actually do not understand that the AppleTV is not, for example, a TV.
2. I don't synch my music. I don't know about 802.11b but the AppleTV has no problem streaming music over 802.11g... even 24-bit Linear PCM.
Additionally, unlike the playback mode when connected to Airport Express, iTunes on the streaming computer doesn't actively "play" the file in the foreground. iTunes only establishes the link with AppleTV to allow the file to be read and bitstreamed directly to AppleTV. There's a significant difference in terms of the resources being used on the streaming computer.
I haven't actually metered the difference in terms of outbound network activity and/or drive activity but Airport Express's method of streaming is considerably more resource-hungry on the computer side because it's playing the file in real-time, then transcoding and transmitting it... some people reported, though I am not certain if this is fact, that it is transmitted as encrypted 320Kbps AAC and then transcoded to Linear PCM (assuming you use the optical output).
The AppleTV has a more powerful internal processor for audio, and a separate one driving the UI, and I suspect that the raw data is bitstreamed directly from a streaming computer to AppleTV and then demuxed and transcoded directly to Linear PCM via the audio chipset on the AppleTV.
To wit... On a G4/933MHz I'm streaming 24-bit Linear PCM to a wireless router over 100baseT ethernet, and from there over 802.11g to AppleTV while doing upsampling DVD rips in the background on the host machine.
Try doing that on the same machine but playing audio over AirTunes to AirPort Express. Not the same deal.