BJ - you're right, the water lines in your house don't roll around hooked to an engine, but neither do the oil ones in these bikes. They move relative to the engine as the tank and engine are bolted to the frame. The tank and engine can move very, very, slightly from each other, but copper is supple enough to handle the vibrations of water pressure changes, the thermal expansion and cooling, and would be just fine in this application.
The bends the builder would need to create would actual absorb the expansion and suppress the changes in distance. Unconventional choice, sure. Imprudent choice, no way.
Me, I'd use stainless tubing to alleviate the potential of leaks or corrosion/oxidation perforating the things, but, copper tubing is cheap enough and widely available that a repair could be done quickly from stock at the home center. Try that with stainless tubing...