Like it or not, technically it is the right call to make. If there is no proactive attempt at storing or possessing this content, then you cannot claim it is possessing the content. Doesnt make kiddie porns existence ok, it is just explaining that there is no proactive and affirmative attempt to store that porn for future use.
Ill give you an example of how the "cache" argument does not work.
If you go looking for porn, you find that many times when you click on one porn site, that suggests that if you click this it will take you to whatever the thumbnail describes. The deeper you go looking for porn the more you see that thumbnails do not always link to what they suggest. Thus many times you can click on a thumbnail based on searching for a innocuous term such as "petite" and get thrown to a mass redirection page with hundreds of thumbnails some of which you had absolutely no intention of clicking, but there they are stored in your cache regardless. You have inadvertently stored literally hundreds of non requested images simply by clicking one link. You as a user can never completely predict what content will be on the other side of a link, so you cannot hold someone liable for a webpage displaying content the user did not intentionally try to access.
EDIT: Example clarification
http://www.dogpile.com/search/images?fcoid=417&fcop=topnav&fpid=27&q=cars&ql=
With this link you see roughly 12 images. Those images are redirected downloads of content from their source pages. Meaning when you looked at images of "cars" you downloaded into your cache content from 15 different webpages with one link, not just one specified webpage. It also clarifies where the source thumbnail originates from. Porn sites typically work in the same manner. So for the purposes of the example, say you only wanted to look up actual cars, and references to Disneys film "Cars" were something prohibited. You simply had no way of controlling based on your search criteria what results would come back, and as you see you get an abundance of "Disneys Cars" results stored in your cache because of the search.
Then there is the deep level of porn where you simply have no way of telling any more. Very few porn links actually affirm the actresses age info or attest to her being over age. There are also fetishes that hover around the area, but are so indistinct that its hard to separate them. School girls, Baby sitters, young, ect. All focused around the same fetish type that portray youth, but are just indistinct enough that you cannot 100% tell as a consumer if the actress is under/over the age of 18 or not. So in many of these cases where you can have someone searching for completely legal but borderline tastes, get exposed to unintended underage content without being able to verify or validate if it is underage or not.
So yes, again, regrettable, but a logical stance given the exact circumstances involved.