1. Seatbelts were mandated in 1967, as far as I recall. I have a '59 Ford with 'optional' seatbelts and had a '65 Ford with aftermarket seatbelts.
2. There are many reasons why one wouldn't want to wear a seatbelt! They may not be apparent to you, but that's because these situations may not fit you. One example is driving on a farm. If I'm at my father-in-law's house, and I want to go over two miles down a private dirt road through a pumpkin field, to my grandmother-in-law's house, I wouldn't put on my seat belt. So in this case, I'd disable the seatbelt buzzer. Another example is if I drive the 1 1/2 mile round trip to my mailbox in a suburban neighborhood, going about 20
mph MAX. There's a pretty good chance I'm not wearing my seatbelt then, either.
3. I don't know of any way this can be done with the Honda, but my Ford has a rather complicated method of turning the buzzer off. I don't recall exactly how to do it (it's in the owner's manual, for you Escape drivers) but it requires turning the car on, buckling up and unbuckling three times, turn the ignition on and off, buckling and unbuckling three more times, then tap on the brakes a set number of times. I suppose they make it difficult to discourage people from turning it off.