As I said, because they didn’t cap the frames in the menu. When this was originally brought up at the Q&A I got the impression they didn’t quite understand what the problem was.
It’s definitely not that. A simple test is to cap the frames and it will stop. No other processes will be affected.
If you write a “busy wait” loop in code then you are effectively doing the same thing except this time it will be the CPU that will be maxed out rather than the GPU. Both the CPU and the GPU can be made to look very busy whilst doing next to nothing.