Ok. What if Amazon customer buys a kindle ebook on Amazon.com and then reads it on iOS Kindle app? I don't imagine Apple gets a cut in that scenario, do they?

Ok. What if Amazon customer buys a kindle ebook on Amazon.com and then reads it on iOS Kindle app? I don't imagine Apple gets a cut in that scenario, do they? A comment here said that Valve was open to disabling buying in the app and they are still banned. Hmm. I can see how Apple's position to get some cut is reasonable, but we are talking multiplatform here. Was Valve unwilling to give Apple any cut at all? Was Apple insisting on too big a cut? What's going on?