Well, I emailed Bill Thompson and he was very forthcoming about his intentions with this piece. It was no art prank and the use of fire was out of respect for the instrument, which was destined for the scrap heap before he rescued it. There were some amazing pictures taken, and the one I found particularly striking showed all the strings flaring out of the instrument after they had popped out. It was a rather beautiful image. Still, some will object to the instrument having been burned. I think that's quite alright; everyone is entitled to their own opinion, I say.
Bill wrote this about the event, which I liked:
"It was truly, honestly, beautiful. I've never seen anything quite like it. I think on the web, or in words, it must sound like a random, violent act or at best an art prank. But it was stunning in its beauty. Several people, several non-artists, even today have remarked on it. It's indescribable."
For those curious, the instrument was a German Sperrhake Passau.