There are actually good reasons for this - the problem is it's very easy to argue against it because any time someone does they do so from the assumption that the person in possession of it is an evil pedophile.
But remember these laws don't discriminate - they not only exist but they generally have mandatory sentences attached. That means a judge is not allowed to take circumstance into consideration. If you have video or images of a minor being abused, you're going to jail.
For example if a mother suspected her child was being abused but the child won't speak up? Maybe she puts a camera in the kids room and catches it the next time it happens. So she takes the footage to the police.... and is immediately arrested herself.
Or perhaps you load up a page from your favourite, legal porn site! Except one of the videos it turns out the 'actors' were 16 year old kids. You didn't watch it, hell you didn't even notice it... but the thumbnail loaded itself to your browser cache. You are now in possession of child porn.
What about when wearable technology really takes off? When we have google glass type devices record everything? What happens when you walk around a corner and see a child being abused... what do you do? Turn the perp in? Oh wait... your recordable device now has child porn on it!
Got a historical book from an era when the age of consent was far lower, that contains illustrations? Child porn.
Discover someone has used your computer to look up child porn? Well too bad because YOU have possession of it now. Don't turn them in because you'll be the one going to jail - scrub the hard drive clean and hope nobody talks to your ISP.
Hell, misclick a link on reddit? Boom, you possess child porn.
And of course we all know what happens when kids take naked photos of themselves and send them to one another. They all apparently go to jail because even though it can be legal for underage kids to have sex, those same people taking photos of themselves is a crime.
The list goes on and on. The abuse of children is a horrible thing and everything that can possibly be done to prevent it should be done. But the laws as they stand need serious adjustment so that the focus is actually toward that goal.