Correlation does not imply causation.
Just because someone smokes crack now and smoked pot before, doesn't mean the pot was the doormat welcoming them to a life of addiction. Not to mention pot does not have any chemically addictive properties.
Again, numerous studies have show that marijuana is *not* a gateway drug. Here's just one study:
http://www.scienceblog.com/cms/study...rug-12116.html
"This evidence supports what’s known as the common liability model, an emerging theory that states the likelihood that someone will transition to the use of illegal drugs is determined not by the preceding use of a particular drug but instead by the user’s individual tendencies and environmental circumstances."