Domestic flights in the US are like busses/trains elsewhere. Most people fly without a checked bag
However I agree that in purely domestic airports I don't see how you'd prevent general public from accessing bags. Except India, wherein you need a booked flight to even enter the airport.
People are routinely prevented from being where they are not supposed to be. Whether you put the baggage pick-up point in a publicly accessible area or on a restricted area is a design choice.
I don't understand this.
Why can't they have a door after the baggage claim that does not permit entry to the baggage claim area?
That's how things work in the UK.
In my local airport, the final part of leaving the arrivals area is the same for both international and domestic flights.
Passport control > Baggage claim (international) > Customs > One-way exit to landside
Baggage claim (domestic) > One-way exit to landside