I was just wondering what the law used in this case was.
A lot of activities are incredibly hard to actually catch someone in the act of, and incredibly hard to actually get hard proof of. Possession, on the other hand, is a strong correlation and proof of itself. The reasonableness of this correlation is what draws the line between whether or not possession is illegal, and that's going to be subjective.
It's really easy to forget that the first and foremost reason we defend privacy isn't because its breach is icky. It's because illegality is sometimes the right thing to do. However, usually it's not. This is a natural contradiction that makes writing law difficult.