Licensing issues?

Android itself is open source, but Google's apps (Gmail, Maps, Market) are not, and manufacturers are not allowed to distribute them without Google's consent.

I'd be surprised if Google was willing to license their proprietary apps (like Gmail) to a competitor like Amazon without requiring the Market app to be preloaded as well. Amazon on the other hand is selling the Kindle Fire more or less at cost and needs to make money by selling content, so preloading Google's Market app on the device is probably the last thing they want to do.