You seem to be saying these restrictions are equivalent, but we're talking about video here, and iTunes is only available on Apple devices and Windows desktop (who wants to watch movies on a desktop computer anyways), whereas Amazon Instant is available on dozens of blu-ray players, smart TVs, set-top boxes, video game consoles that they do not control.
Let's be clear here, iTunes is never coming to any non-Apple device unless the company takes a radical departure from what it has been doing the past 10 years.
Apple is in the business of selling devices and having the most content available to encourage adoption (and doing a pretty good job of it). I don't give a toss that iTunes isn't available on a bunch of Smart TVs or BluRay players, I'm rarely going to buy another BluRay player or Smart TV, nor are most people, they're happy with the one they've currently got.
Amazon is in the content distribution business and playing catchup on having the devices to support it (because, for some reason, having their app on a smart TV and set top box doesn't seem to be enough).
This is a pretty good offering, but it's clear that, HuluPlus and Netflix aside, the big players [Google|Amazon|Apple] wants to have their own device ecosystem, and are fighting to differentiate theirs. For $99, these streaming devices are a disposable income purchase that gives you benefits in a particular ecosystem. Amazon has no particular advantage in this race yet, they're still coming from behind.
No, they do have the advantage of being available on a large number of platforms. This means I can buy a movie from Amazon and feel confident that in 2 years if I decide to buy a different TV, a different set-top box, a different video game console, a different tablet, I probably (but of course not guaranteed) can still watch that movie on it.
Let's be clear here, iTunes is never coming to any non-Apple device unless the company takes a radical departure from what it has been doing the past 10 years.