My original question was how do they know if someone is not fleeing the country IF he can check in at LAX with a CC. The answer: They will check your passport BEFORE you board the flight in EWR. But still, you claim that the airline checks if I have a valid passport, to make sure I can enter the country I am flying to, okay, but don't they ALSO check it to make sure that the people who didn't go through regular security, are not fleeing the country?
And BTW my Dibuk is on the way out
You're mixing up security with border protection.
-TSA (which we all call security) is here to ensure there are no bombs on the flight, and they want to ID you to see that you are actually the person that says on the boarding pass.
-US border patrol is here to ensure that nobody enters the country illegally. They can't hold up anybody that wants to "flee" the country, it's a free country and anybody can do what they want (at least in theory). The truth is that the USA government gets a passenger manifest from every airline, so they can always know who is actually leaving the country if they want to stop that person.
-The airlines check passports to ensure that they don't have to bring you back home if your documents are not in check. Airlines are fined if they bring passengers that can't enter the country + they need to make sure the passenger flies back, which makes the airlines checking your passports and visas (if required).
I hope this clarifies it all.