Essentially many people believe that, because the Bible doesn’t make a direct reference to marijuana that there is no scriptural claim that its wrong.
Faith is complex and misunderstood. Many people might say they have faith in God but most of the time when a person says that what the mean is that they have faith that God exists, that however is very different than having faith in God.
Many people say that the Bible is such an old book that it has no relevance today. Even people whom believe in the Bible as the word of God have trouble relating to the Bible and using it as a practical resource for how to live a Christian life.