Still not seeing this as an antitrust situation. (Just to be clear, I'm going off my understanding of the US specific Sherman Antitrust act). Retail stores change their policies all the time, and don't run afoul of the law, including removing products from sale in said retail establishments. Removal of the products may break contract law depending on the contracts in place, but it's not an antitrust issue. And Apple doesn't have a monopoly position in the tablet space, so this move is not restraining competition or interstate commerce.

The easy way for me to remember antitrust rules is to look back at the Microsoft case. Microsoft obtained monopoly power by becoming the dominant OS for desktop PCs (through other illegal actions, though the monopoly its self wasn't illegal). Microsoft became the only reasonable source of an OS for other businesses independent of Microsoft, such as Dell, Gateway and HP. Microsoft then used their monopoly power to crush Netscape, by both bundling a browser into the OS (still shady, but not the tipping point), and then also forcing said independent companies to not bundle Netscape (the action that violated the competition part of the US antitrust act).