> If the unskilled young men were left in their home country, they would still be unskilled for the rest of their lives.

Most people involved in these groups are either second or third generation children of immigrants who specifically came for work which was available in the late 50's, 60's and early 70's which were specifically targeted since there was a major lack of available manpower due to fast economic growth and reduced workforce (since there was a small war a decade or so ago). Unskilled maybe but they definitely had work.