if smoking marijuana is a sin, why did God create it?
You better read the whole thing before jumping to any conclusions.
This question falsely assumes that marijuana is in and of itself a sin. We must remember that when God created all the plants and vegetation, He pronounced that they were all good (Genesis 1:11-12). Paul, in regards to the what people eat, consume and do with their bodies, twice in 1 Corinthians says that “all things are permissible” (1 Cor 6:12 & 10:23). Jesus Himself had to correct the Pharisees in their thinking that it was what they ate which was so important when He said “What goes into a man's mouth does not make him 'unclean,' but what comes out of his mouth, that is what makes him 'unclean’” (Matt 15:11).
What this means in regards to marijuana is that it is not the plant itself which is sinful, but as Paul continues in both passages in 1 Corinthians is it beneficial and is it in obedience to God? After all, marijuana (like all drugs) may have good, proper, and medical uses in specific circumstances. To say that marijuana is itself sinful is akin to saying that all knives are sinful because they are sometimes used to kill people. It is not the object itself which is sinful, but how the person wields it.
Some people will naturally abuse this theological and say “I can do it as long as I think it’s ok.” This however is also false because it puts you in control of what is ok and what is not. You have no right to determine sin, only God does. Therefore we must ask in the bigger picture what God wants for us, what God created us for, and what Christ redeemed us for. We know that God created us to bear His image (Gen 1:27), to be holy as He is holy (meaning to be set apart and different as God is; Leviticus 19:2), and redeemed us that we might have a life which follows Christ (Romans 7).
It is also to be noted that the Bible urges us to obey our authorities in all manners which do not go against the direct Word of God (Romans 13:1-7 and Acts 5). Therefore because it is illegal and because society will deem it as licentiousness, we must be cautious in doing such an act as smoking marijuana because it will cause others to stumble in either their faith or diminish our witness of the Gospel (1 Corinthians 8:13).
Therefore I can confidently say that there is no one in our congregation of high-school, suburban, Los Angelenos that can say that their act of smoking marijuana is not sinful. However the sin is not the plant itself. The act of smoking marijuana in our particular context is not in obedience to God, it does not glorify His name, it does not represent His holiness to unbelievers, it does not build up or edify the church, it does not show a transformed life, and it does not show a life committed to the fullness of Christ. Smoking marijuana in our context is a selfish, momentary, pleasure-seeking, vain act which puts ourselves as God and voluntarily throws away many of God’s gifts including time, money, our bodies, and self-control. Even if it was prescribed for “medicinal reasons”, there are other ways which can alleviate symptoms which do not compromise the Gospel.
Theologically speaking, a life in Christ technically has the freedom to smoke marijuana, but a life in Christ would have the obedience and love that would rather not.