The Middle east does not care about Immigration as much as other countries would have If they cracked down on Immigration in the middle east It would be harder to traffic arms and drugs to fuel the Terrorist groups right?

This report might put it in perspective for you
http://mmo.gr/pdf/news/Migration_in_the_Middle_East_and_Mediterranean.pdf
Essentially there are two reason for migratory movements in the region
The provision of labour and emigration to Europe and other western countries, the movement of terrorist groups represent a very small number of people