|There is absolutely no steganography in this picture of a fire!|
So the problem with making it so the only way to get from Iraq to the US is being a cooperating asset is that you put our asset's families at risk. We need a huge amount of people who got green cards purely on a lottery or from extended family chains so when we want to offer someone an "expedited magical spy green card" we can, and his/her family won't get automatically kneecapped.
This is one of those strategic dillemas. What if it's 100% true that there's someone bad coming in, because why not? It may literally be impossible to vet people at the border. But if you NEED a permeable border to accomplish building your local HUMINT network, and without one you are completely blind in-country, you may have to just bear that risk?
At some level, building cover traffic is important, and also one of the most difficult things in SIGINT. Keep in mind as far as anyone can tell, public research into stegonography died as soon as digital watermarks clearly were not the answer to DRM for the big media labels - for the simple reason that the way to remove any theoretical digital watermark on a song is to mp3 encode it.