The bigger involvement is indirect I think and was also plainly stated in the quote.
Amazon employs humans, which are potential users of this substance. If using this substance significantly reduces the cognitive ability of their workforce, they'd have an issue.
Amazon (amazon.com side)'s revenue depends on consumer behavior. If marijuana use in the general population is thought to change consumer behavior in their favor, then they would be foolish not to support it. This effect probably outweighs any worker productivity issues that might be introduced.
Amazon employs humans, which are potential users of this substance. If using this substance significantly reduces the cognitive ability of their workforce, they'd have an issue.