The terrorists clearly did not think it was wrong, their opinion was that it is right. Our opinion is that it is wrong. It's opinion.
We have those laws because the majority of people have morals, morals are also opinionistic. Some people have morals which i wouldn't particularly consider.
Almost all countries have laws against marijuana, because people in power consider it wrong... ask a couple of million people if it's wrong however, and they'll say no. It's all opinion really.