Your single example other than Opera is the Kindle app.
Firstly, Apple's own page (http://itunes.apple.com/us/app/kindle/id302584613?mt=8) lists the last update as Apr 21, 2011
Secondly, is there any issue with this app at the moment - I'm not sure why there would be with a reader app (it's rather different to a web browser as the content is 'known' - it doesn't sync pdfs in the way the physical Kindle does)? Frequent updates are fairly irritating once you have a lot of apps on an iOS device. If there's no security risk is there any reason why the gap between updates shouldn't be 6 months, a year, or more?