After reading the full thing, this seems like a reasonable rule to me, rather than trying to enforce it as a black and white law without common sense. It's just a fair leeway for accidentally stumbling across something illegal, vs actively downloading and storing it. Because in this day and age, it is quite possible to accidentally come across things you don't intend to view, and files can stay in the cache for a while. I don't know why your computer would be suspected and searched if this was the case, but at least there is a safety net.
Hopefully they're smart enough to not allow this law to be exploited so someone can to try and store their porn in a cache folder and claim it was an accident, because it would be easy enough to tell the difference.