When you leave the US, you don't go through Passport Control, like you do in most every European country I've had the pleasure to travel to/through. Granted, the airlines take a look at your passport when you check in, but I don't know if they pass any info on to DHS/ICE. I always understood that they are checking to make sure you have valid travel docs so you can actually enter the country you're flying to. To keep them off the hook for knowingly transporting you without the proper docs and facing fines...