StrategyPage has an interesting take on Europe's affinity for terrorists: it's a hostage game.
European nations have long had an "understanding" with Islamic radicals. The Europeans would grant Islamic radicals in general, and their leaders (and their families in particular) sanctuary, as long as there was no terrorist violence in Europe. This arrangement has largely succeeded, if only because the Islamic radical leaders resident in Europe were, in effect, hostages. Start killing Europeans, and we ship you, your wives and kids back to the countries that want to try you for murder and rebellion.I hadn't heard it put quite this way before, but it makes a lot of sense. Ironically for "liberal" Europe, this is a very conservative (as in "safe and historically common") strategy.
Kings and other nobles have long taken mostly-voluntary hostages from each other in the forms of adopted children, wives, and concubines. According to the Bible, King Solomon had seven hundred wives and three hundred concubines (who caused him no end of trouble), largely taken for political purposes. Such hostages were used to cement alliances and enforce subjugation in the days when communication was no faster than a man could walk. If an alliance was broken or a subject nation rebelled the hostages were either killed or used as bargaining chips.
Taking wives and concubines had other obvious benefits as well, since they would bear children of mixed nationality who could be raised loyal/friendly to the king. Such offspring could later be sent back to their mothers' countries to as ambassadors, messengers, or what-not.
Unfortunately for Europe, it doesn't appear that the Arab Muslim populations they're sheltering are assimilating or becoming loyal or friendly to their hosts. Although taking female hostages and slaves was historically common, the women were always married to native men and thus forced to assimilate; Europe's strategy seems to be to give their immigrants free socialized benefits with no strings attached.