Amazon finds a promotional ad unit they like... and they stick to it. I've written before about Amazon effectively selling the "buy / create once, access anywhere" tagline. It's simple and it speaks to: Amazon being more open / compatible than most, and Amazon's content being portable - 'don't lose content you've already purchased'
Here is the promotion for the Kindle Cloud Reader - meaning purchases are accessible via browser, app, and Kindle. And the Cloud Reader itself is accessible anywhere the browser is ... in other words, everywhere:
And here is Amazon's Could Player unit:
Here is the original Kindle promotion: