I had this thought too. As the other comment:

"You can always find ways to put CP on your adversary's computer but it's difficult to make it stick well enough to hold up in court. "

One of the reasons moral =/= legal is that it is not enough that the law is moral, the law must work in practice. If it can not be correctly enforced, it may do more harm than good, and then it may be better to tolerate (*) some evil.

So, the question is not a moral one, it is a legal and technical (network security) issue. Are there cases where child porn was planted into a innocent person's computer? How viable is to do that? How are the courts dealing with that? Do prosecutors need more evidence than 'photos on your computer' to convict? Are cases starting with 'photos on computer' important to start investigations looking for pedo rings?

I don't know the answer. And it may vary on time and place.

(*) that is the definition of 'tolerance'. To fight evil, in practice, with the available means, would create worse evils. So, we abstain from the fight, which is different from accepting it as good.