Amazon's a pretty canny company. Sure, 99% of users who buy a Kindle Fire won't do anything but buy books on it, but that 1% that don't want the UI? Amazon expects you to root the unit, and quickly. PC Magazine got an on the record quote from Jon Jenkins, director of Amazon's Silk browser. He told them that Amazon pretty much expects the Fire to be rooted at some point.
It's going to get rooted, and what you do after you root it is up to you"
This doesn't mean that Amazon's going straight down the HTC-style unlocked bootloader path, however; it's noted in the article that Jenkins didn't know if the bootloader was locked or not, just that the company wouldn't actively do anything to stop the rooting community. [PCMag]