"And a joint IS
but "only" psychically, not physically. Persons with a weak personality shouldn't touch it, as they should not touch either alcohol or coffee. Point is, drugs always existed and will always exist. Imagine the reaction of the german (or belgium) public if their beloved beer was to be forbidden
But, if the kids have to buy their weed from a dealer, they will most likely also get in contact with "harder drugs" (because the dealer has a larger profit margin on them), which could be prevented if they were allowed to buy it legally (either in coffeeshops as in the netherlands or in pharmacys as proposed in germany).
And, one word about the conditioning-drug-theory: It has been proven that there is no causal-connection between marijuana and harder drugs. This theory has often been solely supported by the fact, that most users of hard drugs have been using soft drugs earlier (and if they did, they most likely entered the world of hard drugs because of the reason stated above) The overwhelming majority of soft-drug-users however, has never touched any hard drugs. If there is something like a conditioning drug, then it's alcohol (altough for many, it's already the last drug), or, as it is also considered a drug, coffeine. But somehow nobody sees a connection here.