Though public domain materials are essentially owned by everyone, there are laws requiring people to cite the sources of public domain items they use. As I understand it, the laws vary between countries.
I'm curious about whether these laws are enforced. If someone published an ebook featuring a public domain image with no indication that it's public domain, let alone its source, who would bother suing that person? How frequent are such lawsuits, and what are typical penalties?
To condense this to a single question, I suppose I could just ask "Are such laws enforced?" A link to a reference that explores this topic more thoroughly would be even more helpful.