The "high" of perpetual wars
There is an old maxim which says "war is the health of the state". Unlike some other maxims, this is entirely accurate. No other method permits more expansion of state power against individual freedom than wars - foreign or internal. Ironically, for an organizational structure supposedly fighting against the evils of addiction, government is in fact hopelessly addicted. It is addicted to wars. The War on Drugs is no exception.
Now marijuana, no doubt, is a fine plant which has many legitimate uses. Being extremely averse to most drugs myself, I have no idea of what it's like to smoke marijuana. I'm sure it's relaxing, conductive to socialization, and so on. The medical facts, anyway, indicate clearly that marijuana is inoffensive and may even be beneficial. The medical facts also indicate that alcohol and tobacco, the two most prominent legal drugs, are the drugs most damageable to human life, even if mortality rates themselves are compared.
As for anything else the state does, there is a utilitarian reason for the War on Drugs to attack only certain drugs and not others. These reasons usually start in deep dark history, in some petty corporate or racial dispute, and the perpetuation of the system is inevitably due to the expansion of powers that it gives the state.
As it turns out, drug policies were motivated by racial disputes. Opium was made illegal because Americans feared the power of Chinese immigration, and cocaine was banned because people feared "cocaine-crazed Negroes". The ban on marijuana was based partly on its use by Mexicans, and was also seen as an alternative to the Prohibition that was going on at the time.
The benefits of war to the warring government include the following : higher taxation, general expansion of power, personal glory to politicians, dissent against the state is temporarily silenced, and the creation of new enemies. We observe this in the history of World War I and II, as well as the current American wars. There is, however, one little problem. These benefits only last as long as the threat is feared. Once the war is over, tax rates somewhat drop, although not as low as they were before the war, tbe expansion of power is stopped, and dissent returns.
This is why the state also uses perpetual wars. A perpetual war is a war whose victory conditions are unattainable, usually because the goal is a nebulous concept or for whose opposite there will always be a demand (such as "poverty", "terrorism" or "drugs"). The War on Drugs is one such perpetual war. It is an unwinnable war constructed in order to prop up the state's expansion of power.
Perpetual wars are the state's "high". And when a government gets the munchies, it does not eat chips - but rather human lives and livelihoods.
Remember the maxim ? Its reverse is also true - "the state is the health of war". Without the state, the War on Drugs would likely dissipate. Private individuals would have no incentive to pay for jail space for "victimless crimes", and the lack of strong support for the Drug War would make its persistence unlikely. Furthermore, private individuals would desire the drop in violence that would follow decriminalization, while governments have no such incentive.