When I read that story, my mind went immediately to another MNer, who said not long ago that she wouldn't have a problem with a man stealing her toddler daughter's used nappies for masturbation purposes. Her reasoning was that it wouldn't actually affect her child.
This illustrates exactly why you should have a problem with it.
Sex offenders escalate. Like drug addicts, they start needing a bigger and bigger stimulus to get the same high. A man stealing children's used nappies didn't spontaneously develop a sexual obsession with nappies, and then find himself casting around for a source of nappies. He was obsessed with them because they were related to children, and he wanted to focus on children while masturbating. Eventually indirect contact, like stolen objects won't be sufficient.
I don't know if this man had stolen nappies from another nursery, or whether the nappies he left outside the nursery were all adult ones he'd worn, but between the first time he dumped nappies outside and his capture, he escalated. He enjoyed the idea of others clearing up his excrement, and he wanted them more deeply involved. He went from dumping nappies outside, to hanging around to see the nappies being found, to breaking in just so he could smear his excrement around inside the nursery.