This sort of pre-load / embed solution is lower-risk and easier to execute than a full-on 'Fire Phone', with a completely forked Android and custom hardware: phones are harder than tablets in all sorts of ways. HTC makes it and (with VZW) distributes it, Amazon adds the sizzle.
I saw a job ad that's almost certainly for Amazon the other day: "Head of Strategic Mobile App Partnerships", in Luxembourg, working on "a groundbreaking new mobile platform". More to come, I suspect. This certainly won't be confined to Verizon Wireless in the USA.
I also wonder how long this will be an HTC exclusive. HTC could certainly use the help, but I'd expect all the Android OEMs to be interested: all of them except Samsung are struggling and they have no love for Google. But of course if all Android phones have this then they're back to selling commodities.
And how does Google react? It has the power to withhold access to Google Apps (GMail, Google play, maps etc) for devices that fork Android, but additional preloaded services are hard to fit into that category. Moreover, how soon will Amazon be able to offer all of that functionality itself? Maps are probably the biggest hurdle - but now we have Nokia offering its own (equally good) maps in an iOS and HTML5 solution on all mobile devices...
What is Amazon's strategic objective? This isn't really about selling content and apps on mobile - even Apple makes no money doing that. Rather, as with the Kindle Fire, Amazon is trying to create buying devices. Mobile devices sit next to Vogue and How To Spend It, and on the coffee table in front of the TV. They are ready and waiting for a call to action, to capture purchase intent. Amazon is a leveraged play on the conversion of physical retail to ecommerce, and mobile is the means of acquisition.