Technically the US doesn't prevent you from leaving the country for not having an American passport - it's other countries that are likely not to let you in.
But that's between you and this private company :)
Plus, I think at least part of the reason why airlines would be reluctant to board you is exactly that - knowing that the destination country wouldn't be happy with them dropping you at their doorstep with no passport in hand. What would they care otherwise?
This is exactly it - the airline is responsible for returning you home if you are denied entry to a country.
My wife used to work as an air stewardess, and one of her longer flights was a 16 hour non-stop flight from Dubai to Brazil. This involved a 3 day layover, and on the inbound flight she saw some (rather tired) passengers who had been on the outbound flight.
Apparently they were denied entry, however it was too late for them to go back on the same aircraft, so they had to wait in the airport (airside) for the next flight.
Right, so that goes back to my original point - it's other countries not letting a passport-less person in. The airlines have no other choice but to accept the fact.
An airline denied my passport (had gone through the wash) I had just returned from Canada and had used the passport, in the same condition, in the country I was going to. But the airline didn't care. They said they could be fined $25000 for bringing a passenger without a valid passport plus they would have to return me.
That private company is constrained by rules established in the countries it operates in. It's weirder than you might imagine. I regularly fly between the EU and USA and the immigration and security rules are different for American operators than for their EU counterparts.
