This all fits in with the purpose of a tablet like the Kindle Fire. Bezos was pretty clear during the device's unveiling that its singular purpose is to get you to buy stuff at Amazon, and that the hardware is not supposed to turn a profit in and of itself. The ads, which are displayed on the lock screen, consist of general advertisements, and, of course, special offers from Amazon itself.
In essence, all tablets work this way - to more or lesser degrees - to get you to buy yourself into an ecosystem. The iPad, for instance, comes loaded with stuff like News Stand, which can't be removed or even placed in a folder, and, on top of that, makes it very annoying and cumbersome to buy content from other vendors than Apple. It's not surprising that Amazon would follow suit, but persistent ads on the lockscreen are, of course, way, way worse.
Interestingly enough, the one company from which you'd expect ads plastered all over the place doesn't actually do as such. The Nexus 7 does come loaded with a widget that's a front to the Play Store, but it's easily removed. With the Nexus 7 running Android, you can even switch to a different application store, and there are no restrictions put in place on where and how you can buy content.
Originally, Amazon had no intention of allowing Fire owners to remove the advertisements, but over the weekend, the company reversed this decision. In a statement to The Verge, the company said users will be able to pay a one-time $15 fee to have the ads removed, but that it doesn't expect a whole lot of users to do so. Of course, a much better solution is to simply ditch the Fire's Frankendroid altogether, and switch to CM (whenever it becomes available).
I wouldn't be surprised if we end up seeing more and more of this sort of stuff. The Xbox 360, for instance, is currently plastered with ads all over the place, even for paid Xbox Live Gold subscribers such as myself. The inability to remove crap like this from devices we supposedly "own" is just a further sign that this "post-PC" world so many people gush about is nothing but a change of ownership from user to manufacturer.