That's just the rational thinking, I think, in the absence of good evidence.
As to gateway drugs, the fact that somebody uses marijuana first and then goes on to use something else afterwards, or uses tobacco first and then goes on to use something else afterwards, does not mean that there's a gateway drug phenomenon going on. In fact, we don't even know that there's such a thing as a gateway drug phenomenon. Gateway means that unless you open that gate, the person doesn't go through it and doesn't develop a problem. There's just no evidence for that. It may be the case that a lot of people, before they get into the heavier drugs, will use lighter drugs, if you like, such as alcohol, tobacco or marijuana, but there's not necessarily a causative relationship. What is much more the case is that the people who are driven by their internal discomfort to use any substance at all, are more likely to use other substances in a heavy way or are more likely to use other substances in the same way later on, but this does not mean that the one led to the other. I don't think there's any evidence for basing policy on a gateway drug theory.