There’s this thing with Nigerian Prince spam where spammers are able to send emails to millions of people cheaply, however, the follow-up (phone calls, meeting the people, actually scamming them) is much more expensive. Therefore, the primary goal of spam is to filter out people who are not dumb enough to be scammed. That’s why the emails are dumbed down to the level where only the most gullible will take them seriously.
I wonder whether similar mechanism gets into play with terrorist recruitment. If so, one would expect the propaganda material to be constructed in such a way as to drive away people who are clever, emotionally stable, risk-averse etc.