Even if he had downloaded the images to look at - what harm would it have caused? He didn't ruin some girls life by looking at pictures that already exist.
He didn't ruin the life of the specific girl in the photos. But he incremented the download counter, giving that much more encouragement to the suppliers, letting them know the market was at least one person greater than otherwise.
Maybe after downloading, he was in a conversation where the subject came up and he didn't feel justified in saying, "It's wrong". And so there was one less conversation where it was discouraged. If he justifies it to his own self, the same justification he feels will leak out, just as all the other aspects of his person come through to other people.
All of this adds up to the ruining of the life of a girl - not in the past - but in the future. The next girl.
My comment is hypothetical, because this gent was railroaded, but there exists another fellow for who this does apply. On a macro level, the dynamic holds true. The harm done is that evil propagates itself, and it is worse when it does so in subtle, unquantifiable, yet undeniably real fashion.
What is the justification for (knowingly) having it, and not destroying it? In this case he did destroy it (pay the man respect), and that is why everyone is upset.