When you say, "but we have a constitutional amendment that protects us from self-incrimination", they say "sure but that doesn't apply here."
And of course it is child porn that is in question. It is a mere crime to "possess" it, that is to say, possess a hard drive on which images are found.
It goes without saying that images, at any point in time before, after, or during an investigation, can appear against your will on your hard drive.
But since it is so morally outrageous, it is the go to charge that prosecutors use to jail their personal and political enemies. Or just soft targets, I guess...