It's not surprising that you can't use Amazon Coins for Amazon purchases, but you can't even use them for Kindle books or Amazon Instant Video?
In my opinion, limiting Amazon Coins to Kindle users only would pretty much guarantee them to failure - but Amazon is actually limiting their usefulness to Kindle Fire users only (actually, United States Kindle Fire users only), and while the coins may be fun for people who have already purchased a Kindle Fire anyway, but I can't imagine people being attracted to the Kindle Fire platform just for Amazon Coins, and I can't imagine people being attracted to the Amazon Coins platform for... any reason. ???