See the tagline in the Amazon logo above? They might want to change it to "Use our data — and you're done."
Delicious Library creator Wil Shipley ran afoul of the online book behemoth because his app pulled book info directly from Amazon's Product Advertising API. The API's legal-usage info was recently changed, forbidding "mobile phone[s] or other handheld device[s]" to use the data "without our express prior written approval." Shipley did, in fact, ask permission of Amazon, only to be told there were no exceptions — and they'd pull down his site if he didn't comply.
The assumption here is that Amazon doesn't want competition for its own iPhone app and/or Kindle hardware.