Amazon's Rising Tide

What's that old phrase?...  A rising tide lifts all boats?  That's one of the points made in this Motley Fool article about a recent Amazon promotion.  It's long been suspected that some customers find what they like in a brick-and-mortar store and then go home to order online, from Amazon, for a better discount.  But does the opposite ever happen?  Do very many people see what the like on Amazon and decide instant gratification is more important than 34% off, so they instead buy the book at their local brick-and-mortar store and miss out on the savings?

I'm sure it happens every day, but the Motley Fool example suggests Amazon's promotion of one title in particular is having halo effects elsewhere, specifically at B&N.  It's impossible to examine true cause-and-effect on this sort of thing.  After all, who's to say the book wouldn't have had just as much success at B&N without the Amazon promotion?  Nevertheless, it's an interesting point to consider.  I'll bet it's a model Amazon studies closely as they try to figure out what discount they must offer their customers to prevent a promotion from driving too much traffic/sales to the competition.