My 73 has a seat belt pressure switch under the passenger front seat, it was already unplugged when I pulled the seat to get the broadcast sheet. My guess is, like your saying is that’s it’s annoying and easily defeated. IllI had a 73 Road Runner for 42 years that was purchased in 1979, so it was original except someone had put a 2 barrel carb on it to attempt to get better gas mileage during the gas crisis (wrong move, but that's what they did).
Anyway, my 73 had the lap belts pulled out, wrapped around themselves and then allowed to retract back into the seat belt holder. Once I unwrapped them, I found out why. There is a red seat belt warning sign on the dash along with a buzzer. It stays on and will not stop until the belt is pulled out (and presumably latched). If I recall properly, there was also a passenger seat sensor that would make it do the same thing.
In my case, the buzzer was found and discarded really fast, but I left the light functioning.
Be curious when I do put the seat back in though as to if it functions as designed.