Here is a transcription of my conversation with my Toyota mechanic at the dealership:

Is there any way to turn off that annoying backup beep?

No, sir. That is a safety feature. The car is so quiet that people need to know when it is in reverse.

But the beeping is inside the car, not outside as a warning to others.

It's for your own protection. The car is so quiet that people need to know when it is in reverse.

Yes, but the car is quiet even in drive. Why not a beeping sound then? How about a long high-pitched whistle?

Well, I would hope you know when it is in drive.

By that logic, I should know when it is in reverse. When the car is going backwards, it is in reverse. When it is going forwards, it is in drive.

is a safety feature, sir.