Well i beleive that apple is here to deliver us all the application to make the iphone a universal tool. Porno stuff are more personnal, and with all the adult web application dedicated to the iphone (for example:
http://www.iphonesexy.net ) i don't know why apple store should be invloved in this subject (porn)
In fact, more generally, all the application that connects to the internet are usually "replaceable" with web application. the other applications (database oriented or games) have more their existence legitimated.