Why do some apps only Airplay the audio?

For the background info: I have an iPod Touch [4g] running iOS 5.0.1 and an AppleTV 2 running the latest software.

I've noticed that while the majority of third-party video apps do AirPlay just fine (like Crackle, YouTube, AirVideo, etc.) there are some apps (like CNET TV, and NBA Game Time) that only send over the audio portion of the video to my Apple TV.

In other words, when I try to AirPlay a video it will continue showing the video portion on my local device while playing the audio on the Apple TV.

This seems very strange. I was wondering if this is because:

• I'm doing something wrong?
• The developers of those apps have blocked AirPlay?
• The developers of those apps have not implemented AirPlay properly?
• Or something else entirely?

The developers of those apps have blocked AirPlay video. Though the default is to allow video streaming, individual developers can make that call for their app. Older apps (before iOS 5) will all do it, as well many released after iOS 5 came out.

See the developer documentation, specifically "allowsAirPlayVideo" here.