Even though it would still not be just in a free society:

To ban marijuana, Congress should have amended the Constitution through the arduous process prescribed by the Framers, just as it did when it banned alcohol. Instead it has amended the Constitution through legislative assertion and judicial acquiescence.

Aand, I think a pretty strong case can be made that the alcohol ban was itself unconstitutional.
On a related note Radley Balko argues that the federal medical marijuana ban may in fact be torture.