Then just don't turn your neighbor in when he borrows your car and uses it to transport stolen goods - leaving some of the hot stuff in your trunk.
I was going to say, this will be a tough one to prosecute.
Theft is an act: something committed outside of the brain; something that violates another's personal property rights.
There is no argument that the creators of child-pornography are committing a terrible crime and should be locked up; ratted out instantly; and maybe even flogged daily in the public square.
Even purchasers of stolen goods or child pornography are committing the crime of aiding-and-abetting, which it can be argued, is little different than committing the crime itself.