I agree with Sealman, terrorists do strike for shady reasons. I could envisgage terrorism in Canada, especially if social conflict between British and French speaking Canada increased (not that I am saying it will happen, but it is one way in which Canadian society could split into opposing factions.) Don't think thats realistic in a "moder" society? Go visit northern Ireland or the Basque in Spain.
Also, a religious cult has been responsible for terrorist attacks in Japan. Not your traditional terrorists.
Anyway, terrorism should be random, but affected by certain things. E.g. more unhappy citizens, more likelihood of terrorist attack. More foreign citizens, more likelihood of terrorist attack. Outside fav government, more chance of terrorist attack, in least fav government, even more chance of terrorist attack. Intelligence agency, less chance of terrorist attck, police station, less chance, court, less chance, high culture, less chance.
Remember that terrorist attacks are of a random nature, governments do not always know when they will occur.
I am against player control of terrorist attcks againstother civs. I would recommend players giving money to terrorist groups that are based in other civilizations. That is, each civilizaton would have its own terrorist group. Paying money to terrorist groups could increase the likelihood of attacks. Also, spies could learn that you are funding terrorist groups, leading to strained relationship between you and that civ. Also, intelligence information could be sold to civs for money.