legally, they can't dictate pricing.
Amazon says right on their page for the Macbook:
<b>Retailers like Amazon have the legal right to set their own prices
independently, but some manufacturers place restrictions on how those
prices may be communicated. </b> Because our price on this item is lower
than the manufacturer's "minimum advertised price," the manufacturer
does not allow us to show you our price until you take further action,
such as placing the item in your shopping cart, or in some cases,
proceeding to the final checkout stage. The steps required depend on
the details of the manufacturer's minimum advertised price policy.
Taking these steps allows Amazon to show you our price consistent with
our goal of always offering you the lowest possible prices on the widest
selection of products.