I agree, the Bose A20s are a great product. I have tried literally every other ANR headset and none of them are as comfortable or have as much noise reduction as the A20s. The lack of the auto power on is just a pain or annoying.
Here is the minor problem with your seatbelt analogy from my point of view. In my 7 seat helicopter, I help everyone with their seat belts BEFORE I start the engine. If I could turn on their headphones at the same time I would, but you can't. They can't be switched on until I am buckled in and I apply the avionics power. At that point you have to turn around and teach everyone to find the control panel and tell them which button to turn on. It is not always easy to get them to do this. I fly hundreds and hundreds of new passengers every year in a charter operation and in Young Eagles. I can tell you it is a pain, that I would love to lose. That being said, the A20s are so good otherwise, that I will use them in spite of the auto switching on problem.