Low Voltage AC as power source for Audio Equipment?
Internally it's taking the AC and converting it to DC.  I can't say why, but it's almost certain that it's because they want to have both positive and negative rails, and it's easier to do that if you're getting AC off of the wall-wart.

I suspect that the design (if not the product) is over ten years old, though, because it's gotten so easy to use switching supplies to generate negative voltages, and the switching frequencies are so far beyond audio that the old problem (putting a ca. 20kHz "hum" into your audio) is just not an issue any more.

The only even semi-sensible reason I could see for it not being done is because an older-technology switching supply would be a Very Bad Idea in an audio circuit, and the audio market tends to be quite conservative in what they accept.