"Knowing what I know about computer security and how EASY it is to plant digital evidence without leaving a trace."
This right here is the conversation which needs to happen around this particular topic. In the security community the topic of forensic discovery of child porn tends to get a hand-wave because child porn. It's there so someone must be guilty. The fact is, it is trivial to plant such evidence on anyone's computer and honestly, if a computer is connected to the Internet, define ownership. Is your computer connected to the Internet? Then it's not your computer. Ever used public WiFi? It's absolutely trivial for someone to inject image requests which never get rendered, but which remain in the cache. This can be done en-mass without anyone ever noticing until someone's computer gets searched.
Based on digital evidence alone no one should ever be arrested, let alone convicted on these charges, but the fact is, to catch someone who is guilty is very difficult. Difficult to the degree that it should require video and/or photographic evidence of the perp at the computer looking at the images. Therefore it seems law enforcement and the justice system has decided that the easy way out is to just go with what's on the hard-drive. If a few innocent people get caught, no big deal, it's for the greater good.