As an aside, why is buying from the publisher (the preferred method) almost always more expensive? In this case it's $40 vs $30. Why list it on Amazon for less

As an aside, why is buying from the publisher (the preferred method) almost always more expensive? In this case it's $40 vs $30. Why list it on Amazon for less? Shouldn't the direct from publisher be lower since they don't have to pay Amazon? Plus Amazon throws in free one day shipping. What's the economics behind this?