Steal or profit from Apple's customers? If I am using the kindle app to buy books, then I am Amazon's customer, not Apple's. Apple doesn't own a permanent claim to me just because I bought an iPhone.
That's not what the article is about. The article is talking about Amazon (and other companies too) providing links in apps to their website to purchase other things. That's profiting from Apple's customers. BTW, when you're using Apple's products and downloading apps from their store (whether free or paid) that allow you to buy products from competitors you are still Apple's customer. Don't want to be Apple's customer use an Android phone. You'd still be considered Google's customer.