As an aside, this is something I've only seen in the US. At least in my country the domestic baggage claim area is not accessible unless from an arriving aircraft.
I'm guessing that has more to do with theft though than security.
Do people collect their bags from the baggage claim area and then immediately reboard an aircraft to exit the terminal?
How do the arrivals exist the terminal
Are you not allowed to have a friend who is picking you up assist with baggage claim?
No, that's because in the US they're handling the international flights separately. It's also the reason why even when you have a layover, you need to clear customs.
Domestic flights in the US are like busses/trains elsewhere. Most people fly without a checked bag