There's a more than reasonable reason:
Perhaps Amazon's project froze that portion of the code at some point during 2010. They're shipping v1 of a brand new product based on heavily customized code. At some point, you need to eliminate moving targets if you're ever going to ship.
Amazon's use of Android here is almost an implementation detail. They want to sell you a Kindle so you'll buy content. They're not trying to sell an Android tablet. As a result, I think they're going to sell a ton of them. Enough to make Apple sweat a bit. Very exciting.
Corresponding edit:
>[Edit] Consider JIT (2.2+) and low pause GC (2.3+) that they will be missing. Also consider Flash support which is also 2.2+. Then there is competition from the likes of Archos and you can take their $299 Honeycomb 3.2 tablet and run everything Amazon has to offer on it - Kindle, MP3, Instant video etc.
"No wireless. Less space than a nomad. Lame."