It is trademarked. But it's being challenged. Did you read ANYthing about the suit?
You have very little faith in people who own iPhones/idevices that they would a) go to Amazon's website after seeing an ad for an App on the iPhone. That would be the only scenario, right? Because if you go to the appstore on an idevice - it can ONLY take you to Apple's.
The general public aren't computer experts. A lot of people just google what they are looking for. Someone could be taken to Amazon's app store instead in that case. It was just one example.