This comes down to one thing. Target has come to understand it shouldn't be helping Amazon put Target out of business. Amazon already has an unfair advantage when it comes to online sales in the sense that stores like Target have to charge a sales tax whereas Amazon mostly does not. Companies like Walmart, and presumably Target, have complained about this disadvantage.
Moreover, Amazon uses its monopoly market clout to undermine products stores like Target sell. For instance, Amazon's monopoly with hardcover book sales allows it to force down the cost of e-books below the hardcover books, which in turn puts a hurt on Stores like Targets book sales. It does the same thing with other digital content like music. Moreover, as another person pointed out Amazon has an App that allows users to scan the price of any product in stores like Target to 1) compare the price on Amazon, and 2) let Amazon know what the competitor is charging. Amazon essentially competes with Target in every area of Target's business. Amazon has also gotten around the infamous favored nations clause offered by publishers, by getting exclusive rights to a lot of the most popular literature right now. Take for instance, the Hunger Games or Harry Potter. Odd you can't find this stuff on iBooks.
Apple is less a threat to stores like Target. Apple's real business is selling hardware, not content. Apple's content sales are designed to make its hardware more appealing. On an iPad, you can get a Kindle App, a Target App, a Barnes and Noble app, and the list goes on. Apple wants to control its own destiny so it offers content sales, but it is willing to share content sales to make its devices more attractive. Moreover, to the extent Apple sells content, this is very limited competition to a company like Target, which sells all kinds of products. Barnes and Noble's Nook is essentially in the same category as Apple's iPad in terms of a threat to Target. It is only competing with Target in some limited markets like books, music and movies. This is small potatoes compared to Amazon.