Makes no sense to me. *shrug*
[h/t J.K. for pointing this out -- it really is all about buying and/or licensing the Palm patents:
Here's a more detailed argument in favor of it:
A somewhat cynical perspective on this kind of rumor in general and this rumor in particular.
Basically, this is all discussing what lies between the hardware and the user experience. Could Amazon swap from Android to webOS? Sure. But the argument against is clear (if it works as it is, why switch?) and the argument in favor of it is not. If the Palm business came with enough/the right patents as part of the deal, I could see Amazon paying some amount of money just to increase it's, um, whatever you call a big pile of patents you control when you're sitting at the patent litigation table. But buying a company for the purpose of switching from a functional, shared OS to an as-yet-unproven-in-the-market OS? Those had better be some impressive performance characteristics.
ETA: Oh, I have an idea.
I could see Amazon making a deal with HP if it got Amazon access to the supply chain lined up for the now moribund TouchPad. Access to parts was a huge issue in getting the tablet out this year because everything was already spoken for. The specs on the TouchPad are a great match for Amazon's missing larger tablet. With a $300 BOM for the stripped version of the tablet, Amazon could release a $375-$400 10 inch kFire, possibly in time to really mess with Apple's 2012 early-in-the-year iPad refresh cycle.
If there are indeed a whole bunch of these things in the pipeline that HP is contractually somehow obligated to, I could _definitely_ see Amazon relieving HP of that obligation. What OS would then wind up on it is a whole other question.