Your writing is not very clear (I think you have an "inability" in there instead of an "ability" and a couple of other ambiguous passages).
Anyway, I think you may be asking somewhere in there, "why should porn apps be disallowed from the app store, when porn is generally and readily available on the open internet", and a child presumably has access to a browser?
Basically, there seem to be a couple of misconceptions going on here: 1) that a policy prohibiting porn apps is about nothing more than "controlling what you can and can't do" with your iPhone, iPod or iPad. Certainly, someone who is determined, and maybe even only a little curious, can find it if they want to; or can circumvent the restrictions.
2) That since society is pretty permissive these days and porn is pretty much everywhere you look anyway, why not just throw out all the "pretentious, anachronistic, weird and conservative" notions and just let it all hang out -- why pretend to have any principles or give any heed to others' principles?
Simply put, a parent (myself included) will feel pretty good about such a policy. My kids hijack the iPod Touch and iPad as it is and spend hours on it. It's one thing to know they need some guidance or restrictions while using the internet; at the same time, they don't generally click on ads and they don't put odd and unfamiliar words they don't know how to spell into the search field. However, they will browse the app store and movie trailers much more readily.
Yes, one knows that porn is almost openly displayed on some streets -- that doesn't mean a child should have to come face to face with it in the minimarket when they go to do some shopping. The places that keep it off their shelves are much appreciated by parents. They are showing a little backbone, a little restraint, saying they don't have to participate in a free-for-all to get the attention of jaded consumers; that just because the store owner is in business, it doesn't mean he has to accommodate every flashy but empty industry that would like to squat on his premises and shake down his loyal customers.
Discernment used to be such a highly prized attribute. Now of course, it is denigrated, much as hard work and careful planning and attention to detail are.
