Because

A person can not consistently argue for personal liberties and at the same time be opposed to economic liberty.

[F]or those who can not reconcile the idea that someone who advocates free market capitalism also supports decriminalizing cannabis, consider this – both are expressions of the rights of individuals to live their lives as they see fit. No one else owns your life or what you do with it. That applies equally to the drugs you take and to the productive labour you choose to engage in – and the subsequent disposal of all wealth thus produced.

Read more here.